Create an example custom signer using `hwi-async`

**Describe the enhancement**

The [hwi-async](https://github.com/wizardsardine/async-hwi) crate provides a pure rust interface to various hardware signing devices. It should be possible to create a custom `bdk_wallet` signer using `hwi-async`.

**Use case**

It would be useful to projects using `bdk_wallet` to have an example that uses `hwi-async` as a custom wallet signer.

**Additional context**