Metafield definition reference validations export by handle

Metafield definitions that reference metaobjects can now be transferred between stores.

🚀 What’s New

• Cross-store metafield definitions – validation values for metaobject_reference, list.metaobject_reference, mixed_reference, and list.mixed_reference types now use portable type handles instead of store-specific GIDs.
• Flexible import format – imports accept both type handles (features) and GID format for backwards compatibility.

🔧 Improvements

• Cleaner exports – mixed_reference validations with multiple allowed types now export as comma-separated handles (camera_specs, speaker_specs) instead of JSON arrays.