Instance streaming audit events - add FF and tab

What does this MR do and why?

This MR adds the feature flag instance_streaming_audit_events and when it is enabled it shows the Streams tab in Instance level Audit events page

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the feature flag

   Feature.enable(:instance_streaming_audit_events)

2. Navigate to Admin menu and go to Monitoring -> Audit Events
3. Ensure that the Streams tab is visible.
4. Note: Clicking on this will cause an error due to backend APIs not setup correctly. This will be solved in subsequent MRs

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #398107 (closed)

ee/app/assets/javascripts/audit_events/components/audit_events_app.vue

@@ -13,7 +13,7 @@ export default {
     AuditEventsStream,
   },
   mixins: [Tracking.mixin()],
-  inject: ['isProject', 'showStreams', 'groupPath'],
+  inject: ['isProject', 'showStreams'],
   computed: {
     showTabs() {
       return !this.isProject && this.showStreams;