Use existing bip322 crate

I just noticed there's already a [bip322](https://github.com/rust-bitcoin/bip322) crate. Would it make sense to use that as a dependency rather than re-implementing the same logic in this project?

If you've implemented more features you could also try upstreaming those as PRs on that project.

Start by reading the current message-signing implementation and comparing its behavior with the existing bip322 crate linked in the issue. Check whether the crate covers the project's required features and whether any additions belong upstream. Done means the project uses the dependency where appropriate, with existing behavior preserved and tests passing.

Written by the indexing model from the issue text.