[backport#16944][wallet] add fillPSBT to interface

Summary:
https://github.com/bitcoin/bitcoin/pull/16944/commits/39465d545d521e66bb3accfa788aa94bffaf47eb

Note to reviewers: Original PR has a default SigHashType argument for fillPSBT set in both the declaration and definition, I've removed the default from the definition as it's DRYer and less error-prone that way

Partial backport of Core PR16944

Depends on D7294

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7295