Integrate custom headers deletion 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 delete the custom headers via the GraphQL API

Designs

List	List (before updating is available)	Add stream	Edit stream

Implementation plan

frontend - 2️⃣

Behind the :custom_headers_streaming_audit_events_ui feature flag

1. Update the streaming audit events form to delete the headers on saving the form
2. Add specs for the new deletion logic