Folders: Reduce DB queries when counting and deleting resources under folders (#81153)

* Add folder store method for fetching all folder descendants

* Modify GetDescendantCounts() to fetch folder descendants at once

* Reduce DB calls when counting library panels under dashboard

* Reduce DB calls when counting dashboards under folder

* Reduce DB calls during folder delete

* Modify folder registry to count/delete entities under multiple folders

* Reduce DB calls when counting

* Reduce DB calls when deleting

pkg/services/folder/folderimpl/store_fake.go

@@ -28,7 +28,7 @@ func (f *fakeStore) Create(ctx context.Context, cmd folder.CreateFolderCommand)
 	return f.ExpectedFolder, f.ExpectedError
 }
 
-func (f *fakeStore) Delete(ctx context.Context, uid string, orgID int64) error {
+func (f *fakeStore) Delete(ctx context.Context, UIDs []string, orgID int64) error {
 	f.DeleteCalled = true
 	return f.ExpectedError
 }
@@ -60,3 +60,7 @@ func (f *fakeStore) GetHeight(ctx context.Context, folderUID string, orgID int64
 func (f *fakeStore) GetFolders(ctx context.Context, q getFoldersQuery) ([]*folder.Folder, error) {
 	return f.ExpectedFolders, f.ExpectedError
 }
+
+func (f *fakeStore) GetDescendants(ctx context.Context, orgID int64, ancestor_uid string) ([]*folder.Folder, error) {
+	return f.ExpectedFolders, f.ExpectedError
+}