Add ml to NodeRole enum (#4049)

This commit adds a MachineLearning role to the NodeRole enum. "ml" is now returned as
a node role value in node APIs in 8.0

Fixes #4040

(cherry picked from commit e343b75)

src/Nest/Cluster/NodesInfo/NodeRole.cs

@@ -16,6 +16,9 @@ public enum NodeRole
 		Client,
 
 		[EnumMember(Value = "ingest")]
-		Ingest
+		Ingest,
+
+		[EnumMember(Value = "ml")]
+		MachineLearning
 	}
 }