[Java] Move GCP-related secret classes and tests from sdks/java/core to google-cloud-platform-core

[shunping]

Summary

Moves Google Cloud Platform (GCP) secret manager implementations (GcpSecret, GcpHsmGeneratedSecret) and their SecretRegistrars (GcpSecretRegistrar, GcpHsmGeneratedSecretRegistrar) out of sdks/java/core into sdks/java/extensions/google-cloud-platform-core.
This decouples beam-sdks-java-core from GCP Secret Manager, Cloud KMS, and Tink dependencies, keeping core lightweight while still supporting dynamic secret registration when google-cloud-platform-core is on the classpath via ServiceLoader.

Notice that there is no functional change in this PR.

Follow-up to #39940 (comment)

Changes

• Moved Source Classes:
  • Moved GcpSecret, GcpHsmGeneratedSecret, GcpSecretRegistrar, and GcpHsmGeneratedSecretRegistrar from org.apache.beam.sdk.util (sdks/java/core) to org.apache.beam.sdk.extensions.gcp.util (sdks/java/extensions/google-cloud-platform-core).
• Migrated Tests:
  • Extracted GCP secret unit tests from SecretTest into GcpSecretTest in google-cloud-platform-core.
  • Moved GcpSecretRegistrarTest to google-cloud-platform-core.
  • Extracted GCP KMS and Secret Manager tests for GroupByKey and GroupByEncryptedKey from GroupByKeyTest and GroupByEncryptedKeyTest into GcpGroupByEncryptedKeyTest.
  • Renamed and moved GroupByKeyIT from sdks/java/core to GcpGroupByKeyIT in google-cloud-platform-core.

[shunping]

r: @damccorm

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[damccorm]

Please take a look at the precommit failure which looks related to the change:

1: Task failed with an exception.
  
  Deprecated Gradle features were used in this build, making it incompatible with Gradle 9.0.
  -----------
  * What went wrong:
  Execution failed for task ':sdks:java:core:analyzeClassesDependencies'.
  > Dependency analysis found issues.
    unusedDeclaredArtifacts
     - com.google.protobuf:protobuf-java:4.33.6@jar
  
  
  * Try:
  > Run with --stacktrace option to get the stack trace.
  > Run with --info or --debug option to get more log output.
  > Get more help at https://help.gradle.org./
  ==============================================================================
  
  2: Task failed with an exception.
  -----------
  * What went wrong:
  Execution failed for task ':sdks:java:extensions:google-cloud-platform-core:analyzeClassesDependencies'.
  > Dependency analysis found issues.
    usedUndeclaredArtifacts
     - com.google.protobuf:protobuf-java:4.33.6@jar
     - 
     - ```

[damccorm]

Otherwise, this LGTM, thanks

[shunping]

Note that I also replace the use of GCP secret manager in XVR gbek test with a test secret manager so we won't have gcp deps in java/core. We already have integration tests for GBEK in GcpGroupByKeyIT.java.