Allow advance stage worker to implement its own state checker

For Jira imports we are using a different import state tracker
so we want to let Jira import AdvanceStageWorker to check its
corresponding import state.

app/workers/gitlab/github_import/advance_stage_worker.rb

@@ -21,6 +21,10 @@ module Gitlab
         finish: Stage::FinishImportWorker
       }.freeze
 
+      def find_import_state(project_id)
+        ProjectImportState.jid_by(project_id: project_id, status: :started)
+      end
+
       private
 
       def next_stage_worker(next_stage)

app/workers/gitlab/import/advance_stage.rb

@@ -48,7 +48,7 @@ module Gitlab
       end
 
       def find_import_state(project_id)
-        ProjectImportState.jid_by(project_id: project_id, status: :started)
+        raise NotImplementedError
       end
 
       private

app/workers/gitlab/jira_import/advance_stage_worker.rb

@@ -16,6 +16,10 @@ module Gitlab
         finish: Gitlab::JiraImport::Stage::FinishImportWorker
       }.freeze
 
+      def find_import_state(project_id)
+        ProjectImportState.jid_by(project_id: project_id, status: :started)
+      end
+
       private
 
       def next_stage_worker(next_stage)