Add support for Schnorr signatures

[jbencin]

Currently, Clarity's secp256k1-verify function does not support Schnorr signatures (it ends up calling the Rust function verify_ecdsa()). There is an upcoming use case for Schnorr signatures that I am aware of, it's not urgent, but something we'll want to have

[brice-stacks]

@jbencin can you expand a bit on the design and use case?