Add BLAKE3CONCAT: a version of BLAKE3 that supports chunking

In PR bazelbuild#233 I proposed the addition of two new ContentAddressableStorage
methods (ConcatenateBlobs and SplitBlobs) that allow one to gain random
access it large CAS objects, while still providing a way to very data
integrity. As part of that change, I added a new digest function to help
with that, named BLAKE3CONCAT.

This PR adds just this digest function, without bringing in any support
for chunking. This will be done separately, as it was requested that
both these features landed independently.

I have also included test vectors for the BLAKE3CONCAT digest function.
I have derived these by modifying the BLAKE3 reference implementation
written in Rust, and rerunning the tool that emits the official test
vectors:

https://github.com/BLAKE3-team/BLAKE3/blob/master/test_vectors/test_vectors.json

Furthermore, I have been able to validate the newly obtained test
vectors using a custom BLAKE3CONCAT implementation that I have written
in Go, which will become part of Buildbarn.

Author: EdSchouten

build/bazel/remote/execution/v2/blake3concat_test_vectors.json

@@ -0,0 +1,145 @@
+{
+  "_comment": "Each test is an input length and one hash output. The input in each case is filled with a repeating sequence of 251 bytes: 0, 1, 2, ..., 249, 250, 0, 1, ..., and so on.",
+  "cases": [
+    {
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+}