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.go

@@ -23,8 +23,8 @@ type store interface {
 	// Create creates a folder and returns the newly-created folder.
 	Create(ctx context.Context, cmd folder.CreateFolderCommand) (*folder.Folder, error)
 
-	// Delete deletes a folder from the folder store.
-	Delete(ctx context.Context, uid string, orgID int64) error
+	// Delete folders with the specified UIDs and orgID from the folder store.
+	Delete(ctx context.Context, UIDs []string, orgID int64) error
 
 	// Update updates the given folder's UID, Title, and Description (update mode).
 	// If the NewParentUID field is not nil, it updates also the parent UID (move mode).
@@ -48,4 +48,6 @@ type store interface {
 
 	// GetFolders returns folders with given uids
 	GetFolders(ctx context.Context, q getFoldersQuery) ([]*folder.Folder, error)
+	// GetDescendants returns all descendants of a folder
+	GetDescendants(ctx context.Context, orgID int64, anchestor_uid string) ([]*folder.Folder, error)
 }