Enable annotations query editor

`annotations` field needs to be set in the datasource constructor
in order for annotation query editor to configure itself properly.
See step 2 in docs: https://grafana.com/developers/plugin-tools/create-a-plugin/extend-a-plugin/enable-for-annotations

src/datasource/base.ts

@@ -70,6 +70,7 @@ export class BaseQuickwitDataSource
     this.logLevelField = settingsData.logLevelField || '';
     this.dataLinks = settingsData.dataLinks || [];
     this.languageProvider = new ElasticsearchLanguageProvider(this);
+    this.annotations = {};
   }
 
   query(request: DataQueryRequest<ElasticsearchQuery>): Observable<DataQueryResponse> {