REFERENCE LIBRARY: Adapter Signatures for BIP340 Schnoor for secp256k1

[ChristopherA]

Support for Schnorr adapter signatures

• [ ] Document the minimal changes necessary for the support for adapter signatures in BIP340 Schnorr
• [ ] Review docs with @sipa, etc.
• [ ] propose API changes to @bitcoin-core/secp256k1 master library to support them
  • [ ] try to maintain compatibility with -zkp ecdsa adapter signatures
• [ ] create test cases
• [ ] implement as branch in -staging
• [ ] test and get additional reviewers for -staging version
• [ ] write BIP for upstream master
• [ ] propose PR to upstream master
• [ ] support reviewers
• [ ] PR is accepted by bitcoin-core

Related to #88

/cc @maaku @jesseposner @wolfmcnally @kanzure