Merge bitcoindevkit/bdk#1161: ref(hwi): Move hwi out of bdk

Merge bitcoindevkit/bdk#1161: ref(hwi): Move hwi out of bdk

105d70e9743f651d141c6e0d901d60d583afd4d9 ref(hwi): Move hwi out of bdk (Daniela Brozzoni)

Pull request description:

  Fixes #872

  ### Description

  This commit moves the `hardwaresigner` outside of bdk and inside `bdk_hwi`

  ### Notes to the reviewers

  There are currently two issues with the code:
  - `TransactionSigner` dictates that `sign_transaction` must return a `SignerError` - but being `SignerError` defined inside of bdk, we can't modify it to include an hwi specific error! I don't know how we could fix this (other than getting rid of the trait altogether :)); for now I just added a `SignerError::Generic` variant, lmk if you have better ideas!
  - The hwi tests used the bdk utils to get a funded wallet for testing, which aren't available in `bdk_hwi`, which made me realize - maybe we should expose them so that we can use them across our crates, and also our users can use them to test their code?
  For now, I just left the test commented.

  ### Changelog notice

  - The old `hardwaresigner` module has been moved out of `bdk` and inside `bdk_hwi`.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  ~~* [ ] I've added tests for the new feature~~
  * [x] I've added docs for the new feature

ACKs for top commit:
  notmandatory:
    reACK 105d70e9743f651d141c6e0d901d60d583afd4d9

Tree-SHA512: 9ae3cd035cc27bd7b2831e89c104f40771c6f165cb3bfe1a9052f820050fca7c19f4dd68f171c630a71a7d18ccdee5f94adbc497c6475bd257c2d01cc08109a1