btcec/schnorr/musig2: add key musig2 key aggregation routines
btcec/schnorr/musig2: add key musig2 key aggregation routines

In this commit, we add the set of key aggregation routines for musig2.
This includes the main public key aggregation method, as well as the
aggregation coefficient which is used to compute "mu" when signing.

The logic in this implementation is based on the musig2 paper, as well
as this spec:
https://github.com/ElementsProject/secp256k1-zkp/blob/master/doc/musig-spec.mediawiki.

btcsuite/btcd,tb00/btcd,tb00/btcd,btcsuite/btcd