[backport#15450 2/5] Optionally allow AskPassphraseDialog to output the passphrase

Summary:
https://github.com/bitcoin/bitcoin/pull/15450/commits/60adb21c7affb41ec475a62a53fb0a36bea35dfb

Depends on D7102

Partial backport of Core PR15450

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7103