Feature: Implement PSBT roles

[bbrtj]

I'm looking for volunteers to implement roles defined by PSBT bips. The format is supported, but no code exists which makes automatic operations based on state of the PSBT. This requires implementing roles from BIP174 and BIP370. A likely incomplete list of tasks is gathered below.

• updating and signing a PSBT
• joining multiple PSBTs together
• building a final transaction object based on the contents of a PSBT