Expose method to find key for a single-key destination

Summary: This is a partial backport of Core PR11403 : https://github.com/bitcoin/bitcoin/pull/11403/commits/30a27dc5b18a3ab82a9768710b24d4e7e9661658

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3122