Instance streaming - add headers functionality

What does this MR do and why?

This is a follow up to MR that added streaming destination at the instance level (!119891 (merged) & !119008 (merged)). THis MR enables the custom headers functionality which allows users to add, edit and delete custom headers from streaming destinations. This functionality is already available at Group level https://docs.gitlab.com/ee/administration/audit_event_streaming/index.html#top-level-group-streaming-destinations

Screenshots or screen recordings

Tab-1687314362167

How to set up and validate locally

1. Enable the both FE and BE feature flag

   Feature.enable(:instance_streaming_audit_events)
   Feature.enable(:ff_external_audit_events)

2. Navigate to Admin menu and go to Monitoring -> Audit Events

3. Ensure that the Streams tab is visible.

4. Click on the Streams tab

5. Add a streaming destination with headers

   1. https://docs.gitlab.com/ee/administration/audit_event_streaming/index.html#top-level-group-streaming-destinations

6. Ensure that the headers are added correctly and show

7. Add, edit and delete headers in the destination

8. Ensure that the headers are updated correctly and show

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 #415012 (closed)