Add BLAKE3 hashing algorithm

This uses the single-threaded C-based routines from libblake3.

This is not optimal performance-wise but should be a good starting point
for nix compatibility with BLAKE3 hashing until a more performant
implementation based on the multi-threaded BLAKE3 routines
(written in Rust) can be developed.

src/libutil/experimental-features.hh

@@ -37,6 +37,7 @@ enum struct ExperimentalFeature
     MountedSSHStore,
     VerifiedFetches,
     PipeOperators,
+    BLAKE3Hashes,
 };
 
 /**