Create an example custom signer using `hwi-async`

[notmandatory]

Describe the enhancement

The hwi-async 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

[sdmg15]

Hey @notmandatory ,
Here is my attempt to add the custom signer #206
Let me know if this aligns with what is required (I'm not done yet so it's DRAFT). Thanks

[sdmg15]

Should this issue be mentioned in the list in #70 ?

[notmandatory]

@sdmg15 good idea, I added it to that list.