feat: Referenztypen zwischen Backend und Frontend standardisieren

- `GetReferenceTypes`-Endpunkt im `HistoryController` hinzugefügt, um Key-Value-Paare des `ReferenceType`-Enums im camelCase-Format bereitzustellen.
- `ConfigurationService` in Angular erweitert, um die `ReferenceType`-Werte vom neuen API-Endpunkt abzurufen und zu speichern.

EnvelopeGenerator.GeneratorAPI/ClientApp/envelope-generator-ui/src/app/services/configuration.service.ts

@@ -14,6 +14,8 @@ export class ConfigurationService implements OnInit {
   
   private _envelopeTypeTitles! : any[];
 
+  private referenceType!: any;
+
   constructor(private http: HttpClient) {
     const api_url = inject(API_URL);
     this.url = `${api_url}`;
@@ -25,6 +27,10 @@ export class ConfigurationService implements OnInit {
       this._envelopeTypes = res;
       this._envelopeTypeTitles = res.map(e => e.title);
     }});
+
+    this.http.get<any>(`${this.url}/History/reference-type`).subscribe({
+      next: res => this.referenceType = res
+    })
   }
 
   public get envelopeTypes() {
@@ -34,4 +40,4 @@ export class ConfigurationService implements OnInit {
   public get envelopeTypeTitles() {
     return this._envelopeTypeTitles;
   } 
-}
+}