Build transaction data for updating a SelfMint drop item

Changes an existing self-mint token's metadata and returns the ready-to-sign transaction that points the contract's base URI at the regenerated metadata. It does not change the token's supply. A token id that does not exist on the drop returns 404, and a drop that is not an ERC-1155 self-mint drop returns 400.

An ERC-1155 self-mint drop has no stages, publish step, or public mint. Each item is its own token id: saving one returns a transaction that creates it and mints its whole supply to the contract owner. The transaction's to is the drop's configurer contract (the one its getConfigurer() returns), not the drop contract, and it must be sent from from, the contract owner.

PATCH /api/v2/drops/{slug}/items/{token_id} takes the same body and saves the same metadata, but returns no transaction, so the contract keeps pointing at the old metadata. On a self-mint drop, use this endpoint and send its transaction.