Make SignPSBTInput operate on a private SignatureData object

Summary:
This is a partial backport of Core PR13723 : https://github.com/bitcoin/bitcoin/pull/13723/commits/917353c8b0eff4cd95f9a5f7719f6756bb8338b1

Depends on Depends on D4493

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4494