[MAINTENANCE] Add missing copyright header for Java files

[LantaoJin]

What is the bug?
Following Java files misses the copyright header

/*
 * Copyright OpenSearch Contributors
 * SPDX-License-Identifier: Apache-2.0
 */

• spark-sql-application/src/main/java/org/opensearch/sql/FlintDelegatingSessionCatalog.java
• ppl-spark-integration/src/main/java/org/opensearch/sql/ppl/utils/ParseStrategy.java
• ppl-spark-integration/src/main/java/org/opensearch/sql/ppl/utils/RelationUtils.java
• ppl-spark-integration/src/main/java/org/opensearch/sql/ppl/utils/JoinSpecTransformer.java
• ppl-spark-integration/src/main/java/org/opensearch/sql/common/antlr/Parser.java
• ppl-spark-integration/src/main/java/org/opensearch/sql/ast/tree/Correlation.java
• ppl-spark-integration/src/main/java/org/opensearch/sql/ast/tree/FillNull.java
• ppl-spark-integration/src/main/java/org/opensearch/sql/ast/expression/IsEmpty.java
• ppl-spark-integration/src/main/java/org/opensearch/sql/ast/expression/Scope.java
• ppl-spark-integration/src/main/java/org/opensearch/sql/ast/expression/FieldsMapping.java
• flint-core/src/test/java/org/opensearch/flint/core/metrics/MetricsUtilTest.java
• flint-core/src/test/java/org/opensearch/flint/core/metrics/reporter/DimensionedNameTest.java
• flint-core/src/test/scala/org/opensearch/flint/core/auth/AWSRequestSigningApacheInterceptorTest.java
• flint-core/src/test/scala/org/opensearch/flint/core/storage/OpenSearchBulkRetryWrapperTest.java
• flint-core/src/test/scala/org/opensearch/flint/core/storage/BulkRequestRateLimiterHolderTest.java
• flint-core/src/test/scala/org/opensearch/flint/core/storage/BulkRequestRateLimiterTest.java
• flint-core/src/main/java/org/opensearch/flint/core/metrics/reporter/DimensionedNameBuilder.java
• flint-core/src/main/java/org/opensearch/flint/core/metrics/reporter/DimensionedName.java
• flint-core/src/main/scala/org/opensearch/flint/core/storage/BulkRequestRateLimiter.java
• flint-core/src/main/scala/org/opensearch/flint/core/storage/BulkRequestRateLimiterHolder.java
• flint-core/src/main/scala/org/opensearch/flint/core/storage/OpenSearchUpdater.java
• flint-core/src/main/scala/org/opensearch/flint/core/storage/OpenSearchBulkRetryWrapper.java