Add IRobustRandom extension to get a random value from a data set (#3260)

* Add extension to pick a random element from a dataset

* Add tests

Content.Shared/Utility/SharedRandomExtensions.cs

@@ -0,0 +1,13 @@
+using Content.Shared.Prototypes;
+using Robust.Shared.Random;
+
+namespace Content.Shared.Utility
+{
+    public static class SharedRandomExtensions
+    {
+        public static string Pick(this IRobustRandom random, DatasetPrototype prototype)
+        {
+            return random.Pick(prototype.Values);
+        }
+    }
+}