Build transaction data for creating a SelfMint drop item

Creates one item on an ERC-1155 self-mint drop and returns the ready-to-sign transaction that puts it onchain. Any other drop type 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.

Get media_token from POST /api/v2/drops/{slug}/items/media and upload the file to the returned storage context first. The item is stored and its token id assigned (token_id in the response) before this returns, so calling again creates another item rather than retrying this one; the token is minted only when the transaction confirms.