MuSig2 PSBT Fields.

Adds fields to PSBT files so several people can jointly produce one MuSig2 signature for a shared key.

MuSig2 lets a group combine their keys into one standard key and sign together. It needs extra rounds of messages that the PSBT format, the file passed around for signing, could not hold. These fields carry each person's key, their one-time values and their partial signatures.