Factor out combine / finalize / extract PSBT helpers

Summary:

Refactor the new CombinePSBT, FinalizePSBT, and FinalizeAndExtractPSBT
general-purpose functions out of the combinepsbt and finalizepsbt RPCs,
for use in the GUI code.

Completes backport of core PR14978:
https://github.com/bitcoin/bitcoin/pull/14978/commits/102faad81efa1cb12c29c466cfe81fc8c7351e1d

Depends on D5739.

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5740