feat(backend): Added endpoint to fetch HTML version of learning object (from Dwengo backend)

Also refactored a bit to make this easier.

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

@@ -1,18 +1,27 @@
-import {FilteredLearningObject} from "../../interfaces/learningContent";
+import {
+    FilteredLearningObject,
+    LearningObjectIdentifier,
+    LearningPathIdentifier
+} from "../../interfaces/learning-content";
 
 export interface LearningObjectProvider {
     /**
      * Fetches a single learning object by its HRUID
      */
-    getLearningObjectById(hruid: string, language: string): Promise<FilteredLearningObject | null>;
+    getLearningObjectById(id: LearningObjectIdentifier): Promise<FilteredLearningObject | null>;
 
     /**
      * Fetch full learning object data (metadata)
      */
-    getLearningObjectsFromPath(hruid: string, language: string): Promise<FilteredLearningObject[]>;
+    getLearningObjectsFromPath(id: LearningPathIdentifier): Promise<FilteredLearningObject[]>;
 
     /**
      * Fetch only learning object HRUIDs
      */
-    getLearningObjectIdsFromPath(hruid: string, language: string): Promise<string[]>;
+    getLearningObjectIdsFromPath(id: LearningPathIdentifier): Promise<string[]>;
+
+    /**
+     * Obtain a HTML-rendering of the learning object with the given identifier (as a string).
+     */
+    getLearningObjectHTML(id: LearningObjectIdentifier): Promise<string | null>;
 }