[gui] save PSBT to file
[gui] save PSBT to file

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

Backport of Core [[https://github.com/bitcoin/bitcoin/pull/17509 | PR17509]] part [4/6] : https://github.com/bitcoin/bitcoin/pull/17509/commits/f6895301f768220f3ea70231d5cc5b45ecbf4488

Depends on D8066

Test Plan:
  ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8069

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