extrakeys: Add keypair struct with create, pub and pub_xonly

Summary:
This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/58254463f9a2e96d893157a341c9953c440fdf60

Depends on D7641

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/D7643