Datasources: provide generic function to extract custom headers (#66738)

pkg/services/datasources/fakes/fake_datasource_service.go

@@ -133,3 +133,7 @@ func (s *FakeDataSourceService) DecryptedBasicAuthPassword(ctx context.Context,
 func (s *FakeDataSourceService) DecryptedPassword(ctx context.Context, ds *datasources.DataSource) (string, error) {
 	return "", nil
 }
+
+func (s *FakeDataSourceService) CustomHeaders(ctx context.Context, ds *datasources.DataSource) (map[string]string, error) {
+	return nil, nil
+}