Add "sign message" feature

[tomunchained]

Some users may want to sign a message corresponding to a multisig address. For example, for someone to prove they still hold a key to that quorum without building a transaction.

A possible location for this feature within Caravan would be on the "Addresses" page. Upon clicking on an address, some details and options appear, including Redeem Script, Confirm on Device, UTXOs, etc. Perhaps another option for "Sign Message" could appear here.

[jbrauck-unchained]

Picking this up in my spare time to see what I can do and also get comfortable with the libraries.