Provide a GetApi method on the IScriptEngine to get a named API reference

This allows cross-api method calls on the implementation and also allows
"Meta APIs" that only provide common functionality to other APIs

OpenSim/Region/ScriptEngine/Shared/Instance/ScriptInstance.cs

@@ -714,5 +714,12 @@ namespace OpenSim.Region.ScriptEngine.Shared.Instance
                 throw new Exception("Completed persistence save, but no file was created");
             }
         }
+
+        public IScriptApi GetApi(string name)
+        {
+            if (m_Apis.ContainsKey(name))
+                return m_Apis[name];
+            return null;
+        }
     }
 }