refactor(backend): Preparation for learning content from multiple data sources.

Refactored the service layer so that it becomes possible to add another source for learning objects and learning paths.

backend/src/services/learning-content/learning-object-provider.ts

@@ -0,0 +1,18 @@
+import {FilteredLearningObject} from "../../interfaces/learningContent";
+
+export interface LearningObjectProvider {
+    /**
+     * Fetches a single learning object by its HRUID
+     */
+    getLearningObjectById(hruid: string, language: string): Promise<FilteredLearningObject | null>;
+
+    /**
+     * Fetch full learning object data (metadata)
+     */
+    getLearningObjectsFromPath(hruid: string, language: string): Promise<FilteredLearningObject[]>;
+
+    /**
+     * Fetch only learning object HRUIDs
+     */
+    getLearningObjectIdsFromPath(hruid: string, language: string): Promise<string[]>;
+}