Musig2 support

[Sjors]

BlockstreamResearch/secp256k1-zkp#131 adds MuSig2 support to secp256k1-zkp. According to its documentation the interface is "designed to be misuse resistant". Is there anything libwally-core can add to this? Or should consumers just call the secp256k1 methods directly?

[jgriffiths]

We will wrap the interface for wally no doubt, in order to provide a consistent interface/multi-lang support. I doubt we can make the API any harder to misuse though.