refactor(srp): let universal assets declare native backend keys

managed/XCEngine.ScriptCore/Rendering/Core/ScriptableRenderPipelineAsset.cs

@@ -22,6 +22,11 @@ namespace XCEngine.Rendering
         {
             return FinalColorSettings.CreateDefault();
         }
+
+        protected virtual string GetPipelineRendererAssetKey()
+        {
+            return string.Empty;
+        }
     }
 }
 

managed/XCEngine.ScriptCore/Rendering/Universal/UniversalRenderPipelineAsset.cs

@@ -30,6 +30,11 @@ namespace XCEngine.Rendering.Universal
             }
         }
 
+        protected override string GetPipelineRendererAssetKey()
+        {
+            return "BuiltinForward";
+        }
+
         private ScriptableRendererData ResolveRendererData()
         {
             if (rendererData == null)