Add BLAKE3 hasher to vfs

[tylerwilliams]

Add BLAKE3 hasher to vfs

This PR adds the Blake3Hasher and Blake3HashFunction classes to vfs and
makes them available under the flag --digest_function=BLAKE3.

I spent a little time trying to optimize performance more here, and am happy to
chat about different approaches (ByteBuffer vs byte[] vs unsafe vs mmap). In the
end this CL implements something pretty conservative, but there is room to get
more performance out of this with a different approach.

This is a partial commit for #18658.

Based off of #18731 -- will amend the commit when that is merged.

[tylerwilliams]

(going to edit/resend this)