overview: Provide public access for search provider registration

Provide a reasonably public way to register and unregister search
providers, without adding too much API.

`Main.overview.searchController` provides access that may be generally
useful, while `SearchController.addProvider()` and
`SearchController.removeProvider()` provide a simple interface for
extensions.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2783>

js/ui/overviewControls.js

@@ -871,6 +871,10 @@ class ControlsManager extends St.Widget {
         });
     }
 
+    get searchController() {
+        return this._searchController;
+    }
+
     get searchEntry() {
         return this._searchEntry;
     }