Add Google Cloud Storage (GCS) backend support

[joeybaer]

Summary

Adds GCS backend support alongside the existing S3 backend.

Implementation

• GCS Backend (pkg/backends/gcs.go)

  • Implements Backend interface with full GCS support
  • Uses Application Default Credentials
  • Compatible with Workload Identity Federation for GitHub Actions
  • Stores cache metadata in object custom metadata
  • Properly manages GCS client lifecycle

• Configuration (main.go)

  • Added -gcs-bucket and -gcs-prefix flags
  • Added GCS_BUCKET and GCS_PREFIX environment variables
  • Updated all command help text
  • Added GCS backend case in createBackend()

• Integration Test (integrationtests/integration_gcs_test.go)

  • Full integration test following S3 test pattern
  • Requires TEST_GCS_BUCKET environment variable

• Dependencies (go.mod)

  • Added cloud.google.com/go/storage v1.59.1

Usage

# Using flags
./gobuildcache -backend=gcs -gcs-bucket=my-cache-bucket

# Using environment variables
export GCS_BUCKET=my-cache-bucket
export GOOGLE_APPLICATION_CREDENTIALS=/path/to/key.json
./gobuildcache -backend=gcs

Testing

# Build
go build .

# Integration test
export TEST_GCS_BUCKET=my-test-bucket
go test -v ./integrationtests -run TestCacheIntegrationGCS