feat(blake3): add key derivation

[TamaraFinogina]

PR for Discussion #72.

By design, Blake3 provides a key derivation mode that accepts a context string of any length and key material of any length, producing a derived key of any length. See Blake3 repo.

Hash-wasm didn't have it, but it's easy to add.

P.S. Test vectors are generated by the BLAKE3 team.

[TamaraFinogina]

@Daninet, What do you think?

[Daninet]

I like the idea. Thank you for the PR. I will try to release it with the next version when I have some free time to review the code.