Merge bitcoin/bitcoin#30051: crypto, refactor: add new KeyPair class

ec973dd19719541dbcd6f3a6facf6f5dd7cf439c refactor: remove un-tested early returns (josibake)
72a5822d43d47431b2838ebfcb1f2e21210f5ccb tests: add tests for KeyPair (josibake)
cebb08b121ce8c4c5e68bd043b8668c106e31169 refactor: move SignSchnorr to KeyPair (josibake)
c39fd39ba868253b5118db2e1ac1461d29f0b4ce crypto: add KeyPair wrapper class (josibake)
5d507a0091da1b6c013b00b6c76e19dd4d3b93a7 tests: add key tweak smoke test (josibake)
f14900b6e4eac26ae5f1c0badfa176d895851c97 bench: add benchmark for signing with a taptweak (josibake)

Pull request description:

  Broken out from #28201

  ---

  The wallet returns an untweaked internal key for taproot outputs. If the output commits to a tree of scripts, this key needs to be tweaked with the merkle root. Even if the output does not commit to a tree of scripts, BIP341/342 recommend commiting to a hash of the public key.

  Previously, this logic for applying the taptweak was implemented in the `CKey::SignSchnorr` method.

  This PR moves introduces a KeyPair class which wraps a `secp256k1_keypair` type and refactors SignSchnorr to use this new KeyPair. The KeyPair class is created with an optional merkle_root argument and the logic from BIP341 is applied depending on the state of the merkle_root argument.

  The motivation for this refactor is to be able to use the tap tweak logic outside of signing, e.g. in silent payments when retrieving the private key (see #28201).

  Outside of silent payments, since we almost always convert a `CKey` to a `secp256k1_keypair` when doing anything with taproot keys, it seems generally useful to have a way to model this type in our code base.

ACKs for top commit:
  paplorinc:
    ACK ec973dd19719541dbcd6f3a6facf6f5dd7cf439c - will happily reack if you decide to apply @ismaelsadeeq's suggestions
  ismaelsadeeq:
    Code review ACK ec973dd19719541dbcd6f3a6facf6f5dd7cf439c
  itornaza:
    trACK ec973dd19719541dbcd6f3a6facf6f5dd7cf439c
  theStack:
    Code-review ACK ec973dd19719541dbcd6f3a6facf6f5dd7cf439c

Tree-SHA512: 34947e3eac39bd959807fa21b6045191fc80113bd650f6f08606e4bcd89aa17d6afd48dd034f6741ac4ff304b104fa8c1c1898e297467edcf262d5f97425da7b

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