Add Bindings for Schnorr Adaptor Signatures

[siv2r]

This PR introduces Rust bindings for the Schnorr adaptor signature functionality added in: https://github.com/BlockstreamResearch/secp256k1-zkp/pull/299. The secp256k1-zkp PR is pending review from the maintainers, but I don’t expect the user-facing APIs to change much. Hence, I’m opening the bindings PR here.

Changes:

• Provides FFI interfaces to the corresponding libsecp256k1-zkp functions:
  • secp256k1_schnorr_adaptor_presign
  • secp256k1_schnorr_adaptor_extract
  • secp256k1_schnorr_adaptor_adapt
  • secp256k1_schnorr_adaptor_extract_sec
• Introduces a dedicated module for adaptor signature bindings:
  • adds the zkp::schnorr_adaptor::SchnorrAdaptorPreSignature wrapper type.
  • Implements methods to:
    • Create pre-signatures with presign.
    • Extract the adaptor point via extract_adaptor.
    • Adapt a pre-signature to a valid Schnorr signature using adapt.
    • Extract the secret adaptor through extract_secadaptor.
  • Includes a correctness test to ensure proper functionality.

Note: The overall diff might appear large because the first commit (528f610) updates the secp256k1-zkp-sys vendor with the Schnorr adaptor module (since those changes haven’t been merged upstream yet). The code changes relevant to the bindings are in commits 2 and 3.

[apoelstra]

Nice, thanks!! I reviewed the code, though I don't think we should merge it until upstream merges theirs and we can vendor it from the main repo.