Add simple FlatSigningProvider
Summary: This is a partial backport of Core PR13697 : https://github.com/bitcoin/bitcoin/pull/13697/commits/e54d76044b3a2c625e53f2116c5f6a7c40105d5d
Test Plan:
make check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4360