[gui] load PSBT

Summary:
co-authored-by: Glenn Willen <gwillen@nerdnet.org>

Backport of Core PR17509 part [6/6] : https://github.com/bitcoin/bitcoin/pull/17509/commits/1cd8dc2556b847e11a238b9e69493cd8fbeecc6c

Depends on D8065, D8070, D8068 and D8069

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8071