Proposed BIP for MuSig2 PSBT Fields

I've written a BIP draft for MuSig2 PSBT fields. You can view it on GitHub at https://github.com/achow101/bips/blob/musig2-psbt/bip-musig2-psbt.mediawiki. The draft is based on a gist from Sanket, which can be found at https://gist.github.com/sanket1729/4b525c6049f4d9e034d27368c49f28a6. There are a few notable differences between the draft and the gist. Firstly, the participant pubkeys field is now keyed by only the aggregate xonly key. Secondly, the participant pubkeys themselves are compressed pubkeys instead of xonly.