Unify gcp resource detector

[psx95]

Description:

Remove the dependency on GCP Resource Detection Support Library by inlining the detection code directly in this resource detector.

Testing:

• Existing unit tests for the resource detector pass.
• Included the tests from the detection support library for maintaining test coverage.
• Ran a manual test with a resource detection sample to ensure that the detected resource was not changed.

Documentation:

There should be no user-facing change since the external APIs were not affected.

[Copilot]

Pull request overview

This PR successfully removes the external dependency on the GCP Resource Detection Support Library (com.google.cloud.opentelemetry:detector-resources-support:0.36.0) by inlining all the detection code directly into the gcp-resources module. This is a dependency unification effort that maintains the same public APIs and functionality.

Changes:

• Adds 11 new platform detection classes (GcpPlatformDetector, GcpMetadataConfig, EnvironmentVariables, DetectedPlatform, AttributeKeys, and platform-specific implementations) to replace the external library functionality
• Adds 4 new test support files (TestUtils, GcpPlatformDetectorTest, GcpMetadataConfigTest, EnvVarMock) with comprehensive test coverage from the original library
• Updates GCPResourceProvider and its tests to use the new local classes instead of external imports
• Updates build.gradle.kts to remove the external dependency and add test dependencies (mockito-inline and wiremock-jre8)

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
gcp-resources/src/main/java/.../GcpPlatformDetector.java	New main platform detector class replacing the external library
gcp-resources/src/main/java/.../GcpMetadataConfig.java	New metadata configuration class for fetching GCP instance attributes
gcp-resources/src/main/java/.../EnvironmentVariables.java	New interface for environment variable access abstraction
gcp-resources/src/main/java/.../DetectedPlatform.java	New interface defining the contract for detected platforms
gcp-resources/src/main/java/.../AttributeKeys.java	New attribute keys constants
gcp-resources/src/main/java/.../Google*.java	Platform-specific implementations (GCE, GKE, GAE, Cloud Run, Cloud Functions)
gcp-resources/src/main/java/.../GCPResourceProvider.java	Updated to use local classes instead of external library
gcp-resources/src/test/java/.../GcpPlatformDetectorTest.java	New comprehensive test suite for platform detection
gcp-resources/src/test/java/.../GcpMetadataConfigTest.java	New test suite for metadata configuration
gcp-resources/src/test/java/.../TestUtils.java	New test utility for stubbing endpoints
gcp-resources/build.gradle.kts	Removed external dependency, added test dependencies