Derivation Scheme for MuSig2 Aggregate Keys.

Defines how a group sharing a MuSig2 key can make a fresh shared address for every payment from one extended public key.

Good practice is to use a new address for each payment. Rather than every member combining many keys each time, this turns the group's combined key into a single xpub from which new addresses can be worked out. Anyone holding it can see those addresses, so it needs the same care as any xpub.