Custom Signer

[allisonmoyer]

Describe the enhancement
We'd like to have a custom signer interface similar to https://docs.rs/bdk/latest/bdk/wallet/signer/index.html

Use case
We have a 2-of-3 multisig wallet setup where one key is stored directly on the mobile device. We'd like to store the private key separate from other wallet metadata in a secure store (i.e. Keychain / KeyStore) and only access it when it's needed (for signing) and otherwise not retain it in any way in memory.

Right now to accomplish that, we can maintain one instance of a Wallet object in memory with all 3 public keys and then create a separate Wallet object on demand with 2 public keys and the app private key and use that for signing.

This will work, but it would be better if we didn't have to instantiate an entire Wallet object to accomplish this and rather have access to a customer signer API like what exists at the bdk-core layer. One thing that will be important though is for the ergonomics of using the custom signer to be simple (i.e. just need to provide the raw private key if possible).

We also might be interested in similar functionality for other parts of project (like server signing) cc @alexatblock for more info