[SC-04] Implement transfer_asset function

[yusuftomilola]

Overview

Implement the transfer_asset function that changes ownership of a registered asset inside contracts/contrib/.

Location

All work must be inside contracts/contrib/src/.

Acceptance Criteria

• transfer_asset(env, asset_id, new_owner) function implemented
• Requires auth from current asset owner (current_owner.require_auth())
• Panics if asset not found or status is Retired
• Updates asset.owner to new_owner and asset.last_transfer_timestamp
• Updates owner registry: removes from old owner list, adds to new owner list
• Emits event: topic ["asset", "transferred"], data: asset ID, old owner, new owner