Integrate custom headers list GraphQL into custom headers form for 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

Update the streaming audit events form to list all the custom headers returned by the GraphQL endpoint

Designs

List	List (before updating is available)

Implementation plan

frontend - 2️⃣

Behind the :custom_headers_streaming_audit_events_ui feature flag

1. Update the streaming audit events form to list the custom headers returned by the GraphQL endpoint
2. While the update API is unavailable, the headers should be shown as read-only inputs
3. Add specs for the new list logic