Fixes MixedRealityExtensionServiceAttribute to work on the .NET scripting backend

Types.GetCustomAttributes() doesn't exist on .NET UWP, so we have to use some indirection to get attributes instead.

Author: wiwei

Assets/MixedRealityToolkit/Attributes/MixedRealityExtensionServiceAttribute.cs

@@ -76,7 +76,13 @@ public MixedRealityExtensionServiceAttribute(
         /// </summary>
         public static MixedRealityExtensionServiceAttribute Find(Type type)
         {
+#if WINDOWS_UWP && !ENABLE_IL2CPP
+            // Type.GetCustomAttributes() doesn't exist on UWP .NET, so we have to indirectly use
+            // the TypeInfo instead.
+            return type.GetTypeInfo().GetCustomAttributes(typeof(MixedRealityExtensionServiceAttribute), true).FirstOrDefault() as MixedRealityExtensionServiceAttribute;
+#else
             return type.GetCustomAttributes(typeof(MixedRealityExtensionServiceAttribute), true).FirstOrDefault() as MixedRealityExtensionServiceAttribute;
+#endif
         }
     }
 }