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/Interfaces/IScriptInstance.cs

@@ -85,5 +85,7 @@ namespace OpenSim.Region.ScriptEngine.Interfaces
         DetectParams GetDetectParams(int idx);
         UUID GetDetectID(int idx);
         void SaveState(string assembly);
+
+        IScriptApi GetApi(string name);
     }
 }