Add new endpoint for clients to query registry sparse manifest support:
- GET /api/v1/registry/capabilities returns JSON with sparse_manifests
capability info (supported, required_architectures, optional allowed)
- V2 base endpoint now includes X-Sparse-Manifest-Support and
X-Required-Architectures headers for OCI compliance
- Endpoint accessible without authentication for client discovery
This enables oc-mirror and skopeo to detect sparse manifest support
before attempting filtered multi-architecture mirroring.
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
074c88af13