schnorrsig: Add benchmark for sign and verify

Summary:
This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/8dfd53ee3fa059562483d1867815f78b9e00d947

Depends on D7648

Test Plan:

ninja bench-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7649