Pass a vault's output descriptor to a bitcoin multisig signer

[mgravitt]

Review Spectre Desktop and Caravan to determine which is better for integration.

We will pass the output descriptor (and unsigned/partially signed transaction) to the signer. For this, we will likely need to fork and alter one of the above apps. Caravan seems to be a bit simpler, but I don't think it supports output descriptors yet. Spectre Desktop has a python backend and is more robust, but may also be more complex for integration.