Methods for interacting with PSBT structs

Summary:
Added methods which move data to/from SignatureData objects to
PSBTInput and PSBTOutput objects.

Added sanity checks for PSBTs as a whole which are done immediately
after deserialization.

Added Merge methods to merge a PSBT into another one.

This is a partial backport of Core PR13557 : https://github.com/bitcoin/bitcoin/pull/13557/commits/e9d86a43ad8b1ab83b324e9a7a64c43a61337501

Depends on D4285

Test Plan: Unfortunately, The PR dump all the tests at once at the end, so there are no tests for the new code.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4286