[cryptography] Add Support for Stateless Hash-Based Digital Signatures (SLH-DSA)

[patrick-ogrady]

A few months ago, NIST finalized a standard for hash-based digital signatures based on SPHINCS+ (submitted to the NIST Post-Quantum Standardization effort): https://csrc.nist.gov/pubs/fips/205/final

It would be really cool to add support for this algorithm and even experiment with using STARKs to implement multi-signatures over it (as a Post-Quantum Multi-Signature option). There are some other options that uses lattices (like Dilithium) but I expect those would be a lot slower to run in a STARK (and are a much newer concept).

This will probably require a lot of research to get right but figure its worth posting here for someone to dig into.