A common pattern that MuSig2 functions will use is to aggregate the
pubkeys to get the keyagg_cache and then validate the aggregated pubkey
against a provided aggregate pubkey. A variant of MuSig2AggregatePubkeys
is added which does that.
The functionality of GetMuSig2KeyAggCache and GetCPubKeyFromMuSig2KeyAggCache
are included in MuSig2AggregatePubkeys (and used internally) so there is
no expectation that callers will need these so they are made static.
secp256k1 provides us secp256k1_musig_keyagg_cache objects which we are
used as part of session info and to get the aggregate pubkey. These
helper functions help us convert to/from the secp256k1 C objects into
the Bitcoin Core C++ objects.
