BIP375: Sending Silent Payments in PSBTs

This BIP adds support for sending silent payments using PSBTs.

If there are multiple entities handling the PSBT that do not have access to some input private keys, a DLEQ proof by the signer may be added for other entities to verify the corresponding ECDH shares used to derive the output scripts were generated correctly. This is specified in BIP374. For the common case of a single entity that has access to all private keys, the DLEQ proof generation is unnecessary.

Spending support is trivial and can be done with a modification to BIP370 to add a new input field for the tweak data.