feat(storage): rewrite GCS provider to IStorageProvider parity

[kkopanidis]

Summary

The Google Cloud Storage provider was deprecated and missing critical IStorageProvider methods. This rewrites it to parity with AWS S3 and Azure.

Problems fixed:

• deleteContainer and deleteFolder threw "not implemented"
• getSignedUrl never returned a value
• exists always returned true
• Folder markers used name/keep.txt instead of the shared name.keep.txt convention
• Public access relied on legacy ACLs (makePublic/makePrivate) incompatible with Uniform Bucket-Level Access

What changed:

• Full IStorageProvider implementation with metrics parity (containers_total, folders_total increment/decrement)
• Auth via service account key file, inline JSON (google.serviceAccountKeyJson), or Application Default Credentials
• v4 signed URLs for read/upload and long-lived public URLs on private buckets
• UBLA-safe bucket public access via IAM (roles/storage.objectViewer for allUsers)
• New buckets created with Uniform Bucket-Level Access enabled

Test plan

• Configure storage provider google with a GCS bucket (or fake-gcs-server emulator)
• Auth: verify key file path, inline JSON, and ADC (empty credentials) each initialize successfully
• Container lifecycle: createContainer (public + private), containerExists, setContainerPublicAccess, deleteContainer
• Folder lifecycle: createFolder, folderExists (marker at {folder}.keep.txt), deleteFolder removes marker and prefixed objects
• File ops: store, get, exists (returns false for missing files), delete
• URLs: getSignedUrl returns a working v4 URL; getUploadUrl accepts PUT; getPublicUrl returns direct URL for public buckets and signed URL for private buckets
• Metrics: confirm containers_total / folders_total increment on create and decrement on delete