[secp256k1] Add secp256k1_ec_pubkey_cmp and secp256k1_xonly_pubkey_cmp

Summary:
Allows comparing secp256k1_pubkey and secp256k1_xonly_pubkey respectively, to establish an order between them.

This is a backport of secp256k1#850 and secp256k1#926.

We need this for porting rust-secp256k1 to the secp256k1 library of this repo.

Test Plan: ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16957