Deduplicate SignatureCacheHasher

Summary:
This moves the SignatureCacheHasher to the sigcache header, out of the anonymous namespace, so that the tests can import it.

This is backport of Core's PR9480

Test Plan:

make check

Reviewers: freetrader, CCulianu, #bitcoin_abc, dagurval

Reviewed By: #bitcoin_abc, dagurval

Differential Revision: https://reviews.bitcoinabc.org/D525