Add custom headers form to streaming audit events

Overview

Currently, when streaming audit events, GitLab adds a single HTTP header to aid in verification but group owners cannot specify their own HTTP headers.

Certain ingestion services require certain header values for authentication or other purposes.

Proposal

Add a form to the streaming audit events app to allow users to add up to 20 custom headers.

Designs

List	List (before updating is available)	Max header limit reached	Add stream	Edit stream

Implementation plan

frontend - 3️⃣

1. Add the :custom_headers_streaming_audit_events_ui feature flag
2. Create the designed form with the ability to add and remove rows
   • Limit to 20 rows and show message when reached
   • Automatically show a new blank row when the previous row gets filled in with a header and value
   • Inputs should be read-only when they contain values on the initial setup
   • Make the active checkbox disabled for now
3. Add documentation explaining how to use the new UI and the row limit
4. Add specs for the new UI