Implement Google Cloud Storage-based blob storage

[wadimw]

This PR implements a GCS-backed blob storage backend.

To enable it, specify the following properties:

# Enable GCS client
l10n.gcs.enabled=true

# Use GCS as Blob Storage backend
l10n.blob-storage.type=gcs

# Specify bucket name and object name prefix (i.e. directory)
l10n.blob-storage.gcs.bucket=mojito
l10n.blob-storage.gcs.prefix=mojito

Configuration of the client auth should happen through Application Default Credentials.