NestedFolders: Add library panels counting and deletion to folder registry (#69149)

* Expose library element service's folder service
* Register library panels, add count implementation
* Expand folder counts test
* Update registry deletion method interface
* Allow getting library elements from any folder
* Add test for library panel deletion
* Add test for library panel counting

pkg/services/libraryelements/model/model.go

@@ -230,6 +230,13 @@ type PatchLibraryElementCommand struct {
 	UID string `json:"uid"`
 }
 
+// GetLibraryElementCommand is the command for getting a library element.
+type GetLibraryElementCommand struct {
+	FolderName string
+	FolderID   int64
+	UID        string
+}
+
 // SearchLibraryElementsQuery is the query used for searching for Elements
 type SearchLibraryElementsQuery struct {
 	PerPage          int