Support the official BIP test vectors

Hello, Thanks for the work.
I skimmed through the repository and I can see some of the code with lack of tests (cmiiw).
I think it'll be worth adding more support for the official BIP 322 test vectors, this could strengthen the implementation.
I know it's still experimental maybe that was already on your roadmap, but yeah I'm using this issue to put an URL in it so that we can follow up.

1 - https://github.com/bitcoin/bips/blob/master/bip-0322/generated-test-vectors.json
2 - https://github.com/bitcoin/bips/blob/master/bip-0322/basic-test-vectors.json

Start by reading the linked BIP-322 generated and basic test-vector files, then inspect the repository's current message-signing implementation and tests. Determine which vector cases are already covered and define completion as passing coverage for the applicable official vectors.

Written by the indexing model from the issue text.