AVRO-3918: Add support for UUID encoded as fixed[16] (#2672)

* AVRO-3918: change spec

* Polish

* Include fixed layout to conform to rfc4122

Co-authored-by: Oscar Westra van Holthe - Kind <opwvhk@users.noreply.github.com>

---------

Co-authored-by: Christophe Le Saec <clesaec@talend.com>
Co-authored-by: Oscar Westra van Holthe - Kind <opwvhk@users.noreply.github.com>

doc/content/en/docs/++version++/Specification/_index.md

@@ -825,18 +825,28 @@ _Currently only available in Java and Rust_.
 Here, as scale property is stored in value itself it needs more bytes than preceding `decimal` type, but it allows more flexibility.
 
 ### UUID
+
 The `uuid` logical type represents a random generated universally unique identifier (UUID).
 
-A `uuid` logical type annotates an Avro `string`. The string has to conform with [RFC-4122](https://www.ietf.org/rfc/rfc4122.txt)
+A `uuid` logical type annotates an Avro `string` or `fixed` of length 16. Both the string and `fixed` byte layout have to conform with [RFC-4122](https://www.ietf.org/rfc/rfc4122.txt).
+
+The following schemas represent a uuid:
 
-The following schema represents a uuid:
 ```json
 {
   "type": "string",
   "logicalType": "uuid"
 }
 ```
 
+```json
+{
+  "type": "fixed",
+  "size": "16",
+  "logicalType": "uuid"
+}
+```
+
 ### Date
 The `date` logical type represents a date within the calendar, with no reference to a particular time zone or time of day.