feat(overlay): expose reference to the backdrop element (#9702)

Exposes a reference to the backdrop element via the `OverlayRef` for convenience/customization.

Fixes #9689.

src/cdk/overlay/overlay-ref.ts

@@ -52,6 +52,11 @@ export class OverlayRef implements PortalOutlet {
     return this._pane;
   }
 
+  /** The overlay's backdrop HTML element. */
+  get backdropElement(): HTMLElement | null {
+    return this._backdropElement;
+  }
+
   attach<T>(portal: ComponentPortal<T>): ComponentRef<T>;
   attach<T>(portal: TemplatePortal<T>): EmbeddedViewRef<T>;
   attach(portal: any): any;