Add sign / verify for arbitrary messages

[q-src]

This PR adds support for the creation of signatures of arbitrary messages and their verification. A PR for bdk-kotlin will follow.

Signatures can be created using a given wif, prv or xprv. They can be verified using a pub, xpub or address.

Since these are my very first steps with Rust, please take a closer look at the coding style as well.

[notmandatory]

This looks cool! I'll take a look and it might be something we want to have in bdk eventually.

[thunderbiscuit]

@q-src we've discussed this in 2 dev calls and think it might be valuable to have as part of the core Rust bitcoindevkit. Are you interested in opening a PR to add the feature right in bdk?

Also our dev calls are open and bi-weekly, so if ever that's something you're interested in joining we'd love to have you. You can join our Discord and you'll find all the info there. Cheers!

[q-src]

Thanks for the feedback @thunderbiscuit. I opened a PR at bdk: bitcoindevkit/bdk#601

Sure, I am happy to join the dev calls :)

[thunderbiscuit]

Awesome! You can find the notes and proposed agenda for the upcoming calls both for the core Rust library and the language bindings here.

I keep the agenda for the language bindings evergreen here.

[thunderbiscuit]

Closing in favour of bitcoindevkit/bdk#601.