TF-1809: Handle sync data from cache and network when get email contents

(cherry picked from commit 896059cd5b29c62c2cf34621fe35d7b23a54acf4)

lib/features/email/data/repository/email_repository_impl.dart

@@ -205,7 +205,7 @@ class EmailRepositoryImpl extends EmailRepository {
   }
 
   @override
-  Future<DetailedEmail> getOpenedEmail(Session session, AccountId accountId, EmailId emailId) {
-    throw UnimplementedError();
+  Future<DetailedEmail?> getOpenedEmail(Session session, AccountId accountId, EmailId emailId) {
+    return emailDataSource[DataSourceType.hiveCache]!.getOpenedEmail(session, accountId, emailId);
   }
 }