328, 390, 373: BIPs for MuSig2 derivation, descriptors, and PSBT fields

This PR contains 3 proposed BIPs, the main contents of which were sent to the bitcoin-dev mailing list in October. There have been a few changes, notably the addition of a new BIP for the derivation methodology which was split from the descriptors BIP. I've also changed the PSBT BIP to use 33 byte plain aggregate pubkeys rather than xonly.

Specification