extrakeys: Add keypair_xonly_tweak_add

Summary:
This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/6fcb5b845d2832ce019d60507033f74426290768

Depends on D7643

Test Plan:

cmake -GNinja .. -DSECP256K1_ENABLE_MODULE_EXTRAKEYS=On
ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7644