Move findings deletion into a worker using pub-sub system for `Ci::JobArtifacts::DestroyBatchService`

What does this MR do and why?

This MR introduces event worker using pub-sub to execute security_findings deletion for JobArtifacts::DestroyBatchService to avoid cross database modification.

• Publish event on artifact deletion
• Worker for findings deletion so that we can delete them async

Related to #346236 (closed)

ee/spec/services/ee/ci/job_artifacts/destroy_batch_service_spec.rb

@@ -16,6 +16,23 @@
       expect { subject }.to change { Ci::JobArtifact.count }.by(-1)
     end
 
+    context 'Ci::JobArtifactsDeletedEvent' do
+      let(:event_data) { { job_ids: [artifact.job_id] } }
+
+      it 'is published' do
+        event = double(:event)
+
+        expect(Ci::JobArtifactsDeletedEvent)
+          .to receive(:new)
+          .with(data: event_data)
+          .and_return(event)
+
+        expect(Gitlab::EventStore).to receive(:publish).with(event)
+
+        subject
+      end
+    end
+
     context 'with Geo replication' do
       let_it_be(:primary) { create(:geo_node, :primary) }
       let_it_be(:secondary) { create(:geo_node) }