[FLINK-33914][ci] Introduces a basic GitHub Actions workflow

- Adds basic CI workflow
- Adds composite workflow for starting Flink's CI stages
- Updates CI helper scripts to support GitHub Actions
- Adds composite workflow for doing pre-compile checks
- Adds custom action to enable maven-utils.sh in workflows
- Adds custom action for initializing a job

.github/actions/job_init/action.yml

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+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+#
+---
+name: "Initializes the job"
+description: "Does all the necessary steps to set up the job"
+inputs:
+  jdk_version:
+    description: "The JDK version that's supposed to be used."
+    required: true
+  maven_repo_folder:
+    description: "The location of the local Maven repository (not setting this parameter will omit the caching of Maven artifacts)."
+    required: false
+    default: ""
+  source_directory:
+    description: "Specifies the directory from which the code should be moved from (needed for containerized runs; not setting this parameter will omit moving the checkout)."
+    required: false
+    default: ""
+  target_directory:
+    description: "Specifies the directory to which the code should be moved to (needed for containerized runs; not setting this parameter will omit moving the checkout)."
+    required: false
+    default: ""
+runs:
+  using: "composite"
+  steps:
+    - name: "Initializes GHA_PIPELINE_START_TIME with the job's start time"
+      shell: bash
+      run: |
+        job_start_time="$(date --rfc-3339=seconds)"
+        echo "GHA_PIPELINE_START_TIME=${job_start_time}" >> "${GITHUB_ENV}"
+        echo "The job's start time is set to ${job_start_time}."
+
+    - name: "Set JDK version to ${{ inputs.jdk_version }}"
+      shell: bash
+      run: |
+        echo "JAVA_HOME=$JAVA_HOME_${{ inputs.jdk_version }}_X64" >> "${GITHUB_ENV}"
+        echo "PATH=$JAVA_HOME_${{ inputs.jdk_version }}_X64/bin:$PATH" >> "${GITHUB_ENV}"
+
+    - name: "Setup Maven package cache"
+      if: ${{ inputs.maven_repo_folder != '' }}
+      uses: actions/cache@v4
+      with:
+        path: ${{ inputs.maven_repo_folder }}
+        key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+        restore-keys: ${{ runner.os }}-maven
+
+    - name: "Moves checkout content from ${{ inputs.source_directory }} to ${{ inputs.target_directory }}."
+      if: ${{ inputs.source_directory != '' && inputs.target_directory != '' }}
+      shell: bash
+      run: |
+        mkdir -p ${{ inputs.target_directory }}
+
+        # .scalafmt.conf is needed for Scala format checks
+        # .mvn is needed to make the Maven wrapper accessible in test runs
+        mv ${{ inputs.source_directory }}/* \
+          ${{ inputs.source_directory }}/.scalafmt.conf \
+          ${{ inputs.source_directory }}/.mvn \
+          ${{ inputs.target_directory }}

.github/actions/run_mvn/action.yml

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+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+#
+---
+name: "Runs Maven Command"
+description: "Executes Flink's Maven wrapper with the passed Maven parameters."
+inputs:
+  working_directory:
+    description: "The directory under which the Maven command should be executed."
+    default: "${{ github.workspace }}"
+  maven-parameters:
+    description: "Any parameters of the Maven command."
+    default: ""
+  env:
+    description: "Any environment-specifics that are meant to be available in the context of the call."
+    default: ""
+runs:
+  using: "composite"
+  steps:
+    - name: "Runs Maven Command"
+      working-directory: "${{ inputs.working_directory }}"
+      shell: bash
+      run: |
+        # errexit needs to be disabled explicitly here because maven-utils.sh handles the error if a mirror is not available
+        set +o errexit
+        
+        ${{ inputs.env }} source "./tools/ci/maven-utils.sh"
+        run_mvn ${{ inputs.maven-parameters }}

.github/workflows/ci.yml

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+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This workflow is meant for as a basic CI run covering the most-important features
+# and default Java version. It is meant to run before a PullRequest is merged.
+
+name: "Flink CI (beta)"
+
+on:
+  push:
+  workflow_dispatch:
+
+permissions: read-all
+
+jobs:
+  pre-compile-checks:
+    name: "Pre-compile Checks"
+    uses: ./.github/workflows/template.pre-compile-checks.yml
+  ci:
+    name: "Default (Java 8)"
+    uses: ./.github/workflows/template.flink-ci.yml
+    with:
+      environment: 'PROFILE="-Dinclude_hadoop_aws"'
+      jdk_version: 8
+    secrets:
+      s3_bucket: ${{ secrets.IT_CASE_S3_BUCKET }}
+      s3_access_key: ${{ secrets.IT_CASE_S3_ACCESS_KEY }}
+      s3_secret_key: ${{ secrets.IT_CASE_S3_SECRET_KEY }}