Add Backblaze B2 storage backend

[jeronimodeleon]

Adds a new BackblazeStorage backend using B2's S3-compatible API via boto3 (already a dependency).

Changes

• modelstore/storage/backblaze.py — BackblazeStorage(BlobStorage) class, follows the same pattern as AWSStorage
• modelstore/model_store.py — ModelStore.from_backblaze() classmethod
• modelstore/utils/cli.py — registered in STORAGE_TYPES and MODEL_STORE_TYPES
• examples/examples-by-ml-library/modelstores.py — example function
• tests/storage/test_backblaze.py — mocked unit tests mirroring test_aws.py

B2-specific boto3 config

• SigV4 signing, path-style addressing, disabled auto-checksums
• Custom user agent: b2ai-modelstore

Usage

store = ModelStore.from_backblaze(bucket_name="my-bucket")

Or via environment variables:

MODEL_STORE_B2_BUCKET, B2_APPLICATION_KEY_ID, B2_APPLICATION_KEY