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