Add BIP MuSig2

This PR adds a BIP for the MuSig2 protocol. We, the BIP authors, posted an initial draft version to the bitcoin-dev mailing list in April. Since then, we have addressed feedback from the mailing list and the development git repository (to the best of our knowledge, we didn't leave feedback unaddressed). We kept the resulting change log in the BIP draft. In the past weeks, we received no requests for major changes or features, which indicates that it is a good time to stabilize the BIP draft. The development git repo is archived at https://github.com/jonasnick/bips.

There are already multiple (experimental) implementations of the draft, such as:

• the reference python implementation included in this PR (supports BIP version 1.0.0)
• a libsecp256k1-zkp module (currently supports BIP version 0.1.0)
• btcec (currently supports BIP version 0.4.0)
• secp256kfun (supports BIP version 1.0.0)
• nbitcoin (supports BIP version 1.0.0)

TODO:

• [x] resolve https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-October/021000.html
• [x] after BIP number is assigned, replace alias bip-musig2 with bip-<number>

CC @real-or-random @robot-dreams