feat(appsync): environmentVariables property for GraphqlApi (#29064)

### Reason for this change

AppSync now supports environment variables in GraphQL resolvers and functions. It would be good for `GraphqlApi` construct to have the property.

- https://aws.amazon.com/jp/about-aws/whats-new/2024/02/aws-appsync-environment-variables-graph-ql-resolvers-functions/
- https://docs.aws.amazon.com/en_en/appsync/latest/devguide/environmental-variables.html
- https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-appsync-graphqlapi.html#cfn-appsync-graphqlapi-environmentvariables

### Description of changes

The `environmentVariables` property is added to `GraphqlApi` construct. To add environment variables after the initiation, we can use `addEnvironmentVariables` method.

### Description of how you validated changes

Both unit and integ tests.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk-testing/framework-integ/test/aws-appsync/test/integ.environment-variables.js.snapshot/AppSyncEnvironmentVariables.template.json

@@ -0,0 +1,75 @@
+{
+ "Resources": {
+  "ApiF70053CD": {
+   "Type": "AWS::AppSync::GraphQLApi",
+   "Properties": {
+    "AuthenticationType": "API_KEY",
+    "EnvironmentVariables": {
+     "EnvKey1": "non-empty-1",
+     "EnvKey2": "non-empty-2"
+    },
+    "Name": "Api"
+   }
+  },
+  "ApiSchema510EECD7": {
+   "Type": "AWS::AppSync::GraphQLSchema",
+   "Properties": {
+    "ApiId": {
+     "Fn::GetAtt": [
+      "ApiF70053CD",
+      "ApiId"
+     ]
+    },
+    "Definition": "type Test {\n  id: String!\n  name: String!\n}\ntype Query {\n  getTests: [Test]!\n}\ntype Mutation {\n  addTest(name: String!): Test\n}\n"
+   }
+  },
+  "ApiDefaultApiKeyF991C37B": {
+   "Type": "AWS::AppSync::ApiKey",
+   "Properties": {
+    "ApiId": {
+     "Fn::GetAtt": [
+      "ApiF70053CD",
+      "ApiId"
+     ]
+    }
+   },
+   "DependsOn": [
+    "ApiSchema510EECD7"
+   ]
+  }
+ },
+ "Parameters": {
+  "BootstrapVersion": {
+   "Type": "AWS::SSM::Parameter::Value<String>",
+   "Default": "/cdk-bootstrap/hnb659fds/version",
+   "Description": "Version of the CDK Bootstrap resources in this environment, automatically retrieved from SSM Parameter Store. [cdk:skip]"
+  }
+ },
+ "Rules": {
+  "CheckBootstrapVersion": {
+   "Assertions": [
+    {
+     "Assert": {
+      "Fn::Not": [
+       {
+        "Fn::Contains": [
+         [
+          "1",
+          "2",
+          "3",
+          "4",
+          "5"
+         ],
+         {
+          "Ref": "BootstrapVersion"
+         }
+        ]
+       }
+      ]
+     },
+     "AssertDescription": "CDK bootstrap stack version 6 required. Please run 'cdk bootstrap' with a recent version of the CDK CLI."
+    }
+   ]
+  }
+ }
+}