Implement `ProtocolParticipant` for signing

[marsella]

Addresses part of #425 but doesn't complete it.

This adds everything but the implementation of the actual signing protocol. It

• pulls signature share + signature types into their own module
• adds all the things I think we need to the SignParticipant type (input, participant config, etc)
• implements all of the ProtocolParticipant methods
• Outlines the actual protocol implementation via specific function calls + comments

I'm separating this into its own PR so that it'll be easier to review the signing protocol itself without all this "noise".