[SECP256K1] Extract the secret key from a keypair
Summary:
With schnorrsig if you need to tweak the secret key (for BIP32) you must use the keypair API to get compatible secret/public keys which you do by calling secp256k1_keypair_xonly_tweak_add(), but after that there's no currently a way to extract the secret key back for storage. so I added a secp256k1_keypair_seckey function to extract the key
Backport of secp256k1#845
Test Plan:
ninja check-secp256k1
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D9381