Expose CSignatureCache class in header

Summary:
This is done in preparation for the following commit. Also rename it to
SignatureCache.

This is a partial backport of core#30141
https://github.com/bitcoin/bitcoin/pull/30141/commits/66d74bfc45ae0f743084475ac3bbfb4355bb6ec2
Depends on D18858

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18859