Pay-to-contract tweak fields for PSBT.

Would add fields to PSBT files so signers can spend coins whose keys were adjusted to commit to an extra message.

Some protocols, such as timestamping services, 'tweak' a public key so that it commits to a piece of outside data. To spend those coins later, the signer must apply the same tweak to the private key. This BIP would let a PSBT, the file wallets pass around for signing, carry that information.