refactor(srp): move shadow caster stage selection into managed urp

- add standalone pass asset factories for camera frame stages\n- let managed pipeline assets declare stage pass asset keys\n- make universal renderer data explicitly own the builtin shadow caster stage

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

@@ -58,6 +58,21 @@ namespace XCEngine.Rendering
             return GetPipelineRendererAssetKey();
         }
 
+        protected virtual string GetCameraFrameStandalonePassAssetKey(
+            CameraFrameStage stage)
+        {
+            return string.Empty;
+        }
+
+        protected virtual string
+            GetCameraFrameStandalonePassAssetKeyContextual(
+                CameraFrameStage stage,
+                int rendererIndex)
+        {
+            return GetCameraFrameStandalonePassAssetKey(
+                stage);
+        }
+
         protected virtual void ReleaseRuntimeResources()
         {
         }