Separate helper function for ec_pubkey_tweak_add

Summary:
This is in preparation for allowing code reuse by xonly tweak add functions

This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/176bfb1110147b5dca1834ea071acc846fb1cab3

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7640