Allow initializing tagged sha256

Summary:
This will be used by the schnorrsig module
This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/eabd9bc46a31c0da6db6d88840eadbe9006447b1

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7645