Integrate custom headers update 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 save mutations to headers via the GraphQL API

Designs

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 update the existing headers on saving of the form
2. Remove read-only limitation on inputs and remove the disabled attribute from the active checkbox
3. Add specs for the new mutation logic
4. Update specs for the inputs and checkbox