[backport#15427 2/3] Abstract out UpdatePSBTOutput from FillPSBT

Summary:
https://github.com/bitcoin/bitcoin/pull/15427/commits/3135c1a2d2e2fb31bc362c848bd2456d576e408b

Depends on D6616

Partial backport of Core PR15427

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6624