Add blake3 as a registered/supported hash algorithm.

Motivations for this PR:
1. Blake3 is a high performance hash and there is growing community
   interest
2. Blake3 is variable output but mandate 256-bit output

Signed-off-by: Ramkumar Chinchani <rchincha.dev@gmail.com>

descriptor.md

@@ -95,6 +95,7 @@ Some example digest strings include the following:
 | `sha512:401b09eab3c013d4ca54922bb802bec8fd5318192b0a75f201d8b372742...`   | [SHA-512](#sha-512)         | Yes        |
 | `multihash+base58:QmRZxt2b1FVZPNqd8hsiykDL3TdBDeTSPX9Kv46HmX4Gx8`         | Multihash                   | No         |
 | `sha256+b64u:LCa0a2j_xo_5m0U8HTBBNBNCLXBkg7-g-YpeiGJm564`                 | SHA-256 with urlsafe base64 | No         |
+| `blake3:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b` | [BLAKE3](#blake3)           | Yes        |
 
 Please see [Registered Algorithms](#registered-algorithms) for a list of registered algorithms.
 
@@ -161,6 +162,11 @@ Implementations MAY implement SHA-512 digest verification for use in descriptors
 When the _algorithm identifier_ is `sha512`, the _encoded_ portion MUST match `/[a-f0-9]{128}/`.
 Note that `[A-F]` MUST NOT be used here.
 
+#### BLAKE3
+
+[BLAKE3][blake3] is a high performance, highly parallelizable, collision-resistant hash function which [is more performant][blake3-vs-sha2] than
+[SHA-256][rfc4634-s4.1]. The hash output length MUST be 256 bits.
+
 ## Embedded Content
 
 In many contexts, such as when downloading content over a network, resolving a descriptor to its content has a measurable fixed "roundtrip" latency cost.
@@ -220,3 +226,5 @@ In the following example, the descriptor indicates the type of artifact it is re
 [rfc7230-s2.7]: https://tools.ietf.org/html/rfc7230#section-2.7
 [sha256-vs-sha512]: https://groups.google.com/a/opencontainers.org/forum/#!topic/dev/hsMw7cAwrZE
 [iana]: https://www.iana.org/assignments/media-types/media-types.xhtml
+[blake3]: https://www.ietf.org/archive/id/draft-aumasson-blake3-00.html
+[blake3-vs-sha2]: https://github.com/BLAKE3-team/BLAKE3-specs/blob/master/blake3.pdf