Since we explicitly keep an old version of a library in lnd for backward
compatibility we want to make sure the purpose and version of it is
clear and not misleading.
With this commit we copy the exact code of the MuSig2 code as found in
github.com/btcsuite/btcec/v2/schnorr/musig2 at the tag btcec/v2.2.2.
This corresponds to the MuSig2 BIP specification version of v0.4.0.