Add GCS Support

[thespags]

From @avdasCrusoe Description:

This PR adds support for using Google Cloud Storage as a remote cache backend alongside the existing S3 support.

Key Changes

New Components

• gcsutil package: GCS client wrapper for operations
• GCSCache implementation: Build cache backed by GCS
• Storage abstraction layer: CacheClient interface supporting both S3 and GCS
• Adapter pattern: S3Adapter and GCSAdapter implementing unified interface

Command-Line Interface

• --storage - Choose backend: s3 or gcs
• --gcs-bucket - GCS bucket name
• --gcs-key-file - Path to GCS service account key file
• --gcs-concurrency - Maximum concurrency for GCS uploads

Code Organization

• Renamed lib/gobuild/gobuild.go → lib/gobuild/s3.go for clarity
• Refactored modproxy and revproxy to use storage abstraction

Implementation Details

The GCS implementation follows the same patterns as S3:

• Concurrent uploads with configurable concurrency
• ETag-based duplicate detection to avoid redundant uploads
• Minimum upload size threshold support
• Comprehensive metrics for cache hits/misses/uploads

Backward Compatibility

• All existing S3 functionality preserved
• Existing S3 flags unchanged (--s3-bucket, --s3-region, etc.)
• No breaking changes to existing deployments

Testing

• Builds successfully with both S3 and GCS backends
• Tested with real GCS bucket and workloads
• Metrics reporting working correctly