Add a custom signer for hardware wallets

[wszdexdrf]

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Also adds a new test in CI for building and testing on a virtual hardware wallet.

Description

This PR would enable BDK users to sign transactions using a hardware wallet. It is just the beginning hence there are no complex features, but I hope not for long. I have added a test in CI for building a ledger emulator and running the new test on it. The test is similar to the one on bitcoindevkit/rust-hwi.

Notes to the reviewers

The PR is incomplete (and wouldn't work, as the rust-hwi in cargo.toml is pointing to a local crate, temporarily) as a small change is required in rust-hwi (https://github.com/bitcoindevkit/rust-hwi/pull/42).

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing

New Features:

• [x] I've added tests for the new feature
• [x] I've added docs for the new feature
• [x] I've updated CHANGELOG.md

[evanlinjin]

This is exciting that rust-hwi is finally getting integrated into BDK :)

I'm wondering whether "Hardware Wallet" is the best name since we have a struct named Wallet, and there is also a portion of the Bitcoin community wary of the term "Hardware Wallet" anyway. The proposed alternative term is "Hardware Signer" and since our signing traits are already called {}Signers, I think this may be a better term.

[wszdexdrf]

I'm wondering whether "Hardware Wallet" is the best name since we have a struct named Wallet

Now that you say it, it makes sense. This isn't anywhere close to the Wallet struct in terms of functionality... so maybe Hardware Signer is better...

[danielabrozzoni]

I've just published hwi 0.2.1 btw :)

[danielabrozzoni]

Btw, I don't like that much having the hw module under src/hardwaresigner, it feels out of place... I'd say it should be either in src/wallet/signer.rs, or maybe you could make signer.rs a module and put the hardwaresigner in src/wallet/signer/hardwaresigner.rs, as you wish