Add BLAKE3 hash function

[ethindp]

Would it be possible to add the BLAKE3 hash function? BLAKE3 is a very fast, parallelizable hash function that is a sort-of successor to the BLAKE2 family of hash functions. We could either import the official code (https://github.com/BLAKE3-team/BLAKE3) or Botan could have it's own.

[randombit]

BLAKE3 is in scope. It would likely have to be a completely new implementation. Partially for licensing reasons but really more because Botan has its way of doing things [*] and in general the cases where we've imported some third party code where the license was compatible, and not rewritten it to match our approach, it's caused problems for future maintenance and I've regretted it.

[*] Especially relevant in this case: std::span wherever possible, no asm at all unless it's critical (BLAKE3 doesn't count for this imo and it really shouldn't be necessary anyway), SIMD is wherever possible using generic wrappers that handle multiple SIMD ISAs in one implementation.