Move FillPSBT to be a member of CWallet

Summary: Backport of Core PR18115 part [5/9] : https://github.com/bitcoin/bitcoin/pull/18115/commits/3d70dd99f9f74eef70b19ff6f6f850adc0d5ef8f

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8097