Add a CHashWriter::GetSHA256() method to compute a single sha256

Summary:
This is a partial backport of core#19601

The unbackported part of the PR is currently only used by Bitcoin Core for Taproot related code, so I don't excpect we will need it. The backported method is needed in the MuHash3072 implementation.

The test vectors are from https://www.di-mgt.com.au/sha_testvectors.html

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11149