Add pczt frost-sign

[conradoplg]

This is just a proof of concept to gather feedback. The code is ugly; I'm looking for feedback on (1) is the API something acceptable? and (2) is this something that could be possibly merged after cleaning up?

Regarding the API, currently it is supposed to work along the frost-client CLI for the participants (and the frost-server in the same repo to handle comms), but if we want to go ahead it might make sense to add a new command for participants (pczt frost-sign does the coordinator logic) so that using frost-client is not required.

Regarding what is currently missing:

• Handling multiple inputs
• Sending the PCZT for the participants to validate (critical!)
• Deciding how to handle FROST key material (currently uses the frost-client config file which stores everything in cleartext)
• Incorporating ZIP-312 when that is done