Build creator fee enforcement transactions

Build the transactions that turn creator fee enforcement on or off for a collection, for the owner to sign and send. Turning it on sets OpenSea's transfer validator on the contract (the Creator earnings tab in Studio); turning it off removes it. Nothing changes until the transactions are mined. Only the collection owner can call this, and the contract must be owned onchain by that wallet. When enforcement is already in the requested state, transactions is empty and there is nothing to send. Check the result with GET /api/v2/collections/{slug}/creator_fee_enforcement.