Add optional submitter fee enforcement

[Arinerron]

Idea: in the future, if you had your commitments/salts but didn't want to deal with funding an account with native coin, you could use a "submitter" who submits the nullifier onchain then makes a call to allow the sender to transfer fees to the submitter (can verify it's profitable offchain first). Can have a smart contract onchain that ensures the submitter's balance goes up too.

low priority / need to think on it more

[Arinerron]

This could be done with a separate contract, where you also submit a proof that the submitter has been compensated in one of the output commitments.