feat(headlines): add similar headlines event

- Added FetchSimilarHeadlines event
- Included currentHeadline property

Author: fulleni

lib/headline-details/bloc/similar_headlines_event.dart

@@ -0,0 +1,17 @@
+part of 'similar_headlines_bloc.dart';
+
+abstract class SimilarHeadlinesEvent extends Equatable {
+  const SimilarHeadlinesEvent();
+
+  @override
+  List<Object> get props => [];
+}
+
+class FetchSimilarHeadlines extends SimilarHeadlinesEvent {
+  const FetchSimilarHeadlines({required this.currentHeadline});
+
+  final Headline currentHeadline;
+
+  @override
+  List<Object> get props => [currentHeadline];
+}