refactor(srp): move directional shadow execution policy into managed urp

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

@@ -73,6 +73,19 @@ namespace XCEngine.Rendering
                 stage);
         }
 
+        protected virtual string
+            GetDirectionalShadowExecutionPolicyAssetKey()
+        {
+            return string.Empty;
+        }
+
+        protected virtual string
+            GetDirectionalShadowExecutionPolicyAssetKeyContextual(
+                int rendererIndex)
+        {
+            return GetDirectionalShadowExecutionPolicyAssetKey();
+        }
+
         protected virtual void ReleaseRuntimeResources()
         {
         }