Initial release with Google Cloud Storage (GCS) cache backend support:
## Added
- Google Cloud Storage (GCS) support as a cache backend with automatic fallback to GitHub's built-in cache
- \`gcs-bucket\` parameter to specify the GCS bucket for caching
- \`gcs-path-prefix\` parameter to specify a custom prefix within the GCS bucket
- Comprehensive documentation for GCS integration
## Benefits
- Store caches beyond GitHub's 10GB repository limit
- Share caches across multiple repositories
- Custom retention policies through GCS lifecycle management
- Cross-branch cache sharing capabilities
## Compatibility
- Maintains full compatibility with the original GitHub cache action API
- Based on actions/cache@v4 with all existing features preserved
