Add Google Cloud OpenTelemetry plugin

[eamsden]

What was changed

• Added the temporal-gcp module and GcpOpenTelemetryPlugin as a thin Google Cloud Run configuration layer over temporal-opentelemetry.
• Added Cloud Run worker-pool and service naming defaults while retaining standard OTLP endpoint overrides and worker shutdown flushing.
• Added focused tests, Gradle module wiring, and a Temporal BOM constraint.
• Added module documentation that makes the required collector sidecar explicit and links to the Google-Built OpenTelemetry Collector deployment guidance.

Why?

Temporal Java workers running on Google Cloud Run need an SDK-supported path for exporting Temporal metrics and traces through the standard local OTLP collector sidecar. Keeping Google resource detection and exporters in the collector avoids Google Cloud client dependencies and direct-export configuration in the worker process.

Checklist

1. Closes: N/A.

2. How was this tested:

   • ./gradlew :temporal-gcp:build :temporal-bom:generatePomFileForMavenJavaPublication --offline
   • Six focused unit tests cover configuration precedence, plugin installation, and shutdown flushing.
   • Verified the generated temporal-gcp POM only publishes temporal-opentelemetry as a dependency.

3. Any docs updates needed?

   • Added contrib/temporal-gcp/README.md with setup, precedence, lifecycle, and collector-sidecar requirements.

Breaking changes?

No.

Server PR

None.