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 - :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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