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-service.ts

@@ -1,5 +1,14 @@
-import {FilteredLearningObject} from "../../interfaces/learningContent";
+import {
+    FilteredLearningObject,
+    LearningObjectIdentifier,
+    LearningPathIdentifier
+} from "../../interfaces/learning-content";
 import dwengoApiLearningObjectProvider from "./dwengo-api/dwengo-api-learning-object-provider";
+import {LearningObjectProvider} from "./learning-object-provider";
+
+function getProvider(id: LearningObjectIdentifier): LearningObjectProvider {
+    return dwengoApiLearningObjectProvider
+}
 
 /**
  * Service providing access to data about learning objects from the appropriate data source (database or Dwengo-api)
@@ -8,22 +17,29 @@ const learningObjectService = {
     /**
      * Fetches a single learning object by its HRUID
      */
-    getLearningObjectById(hruid: string, language: string): Promise<FilteredLearningObject | null> {
-        return dwengoApiLearningObjectProvider.getLearningObjectById(hruid, language);
+    getLearningObjectById(id: LearningObjectIdentifier): Promise<FilteredLearningObject | null> {
+        return getProvider(id).getLearningObjectById(id);
     },
 
     /**
      * Fetch full learning object data (metadata)
      */
-    getLearningObjectsFromPath(hruid: string, language: string): Promise<FilteredLearningObject[]> {
-        return dwengoApiLearningObjectProvider.getLearningObjectsFromPath(hruid, language);
+    getLearningObjectsFromPath(id: LearningPathIdentifier): Promise<FilteredLearningObject[]> {
+        return getProvider(id).getLearningObjectsFromPath(id);
     },
 
     /**
      * Fetch only learning object HRUIDs
      */
-    getLearningObjectIdsFromPath(hruid: string, language: string): Promise<string[]> {
-        return dwengoApiLearningObjectProvider.getLearningObjectIdsFromPath(hruid, language);
+    getLearningObjectIdsFromPath(id: LearningPathIdentifier): Promise<string[]> {
+        return getProvider(id).getLearningObjectIdsFromPath(id);
+    },
+
+    /**
+     * Obtain a HTML-rendering of the learning object with the given identifier (as a string).
+     */
+    getLearningObjectHTML(id: LearningObjectIdentifier): Promise<string | null> {
+        return getProvider(id).getLearningObjectHTML(id);
     }
 };