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 | | ---- | ----------------------------------- | ------------------------ | ---------- | ----------- | | ![image](https://gitlab.com/groups/gitlab-org/-/uploads/bdaeea23d15914af8087397514ac5df5/image.png) | ![image](https://gitlab.com/gitlab-org/gitlab/uploads/9d17bd76cedad0981eae5dca25941e31/image.png) | ![image](https://gitlab.com/gitlab-org/gitlab/uploads/06217beeda2a4df40bf86fdcb668ef15/image.png) | ![image](https://gitlab.com/groups/gitlab-org/-/uploads/7bc79fdcf3319d2b368b82d72078f79b/image.png) | ![image](https://gitlab.com/groups/gitlab-org/-/uploads/7bc79fdcf3319d2b368b82d72078f79b/image.png) | ## Implementation plan ~frontend - :three: 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
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