Add ability to activate/deactivate headers using the UI

Overview

Now we have the custom headers GraphQL API, we want to allow the user to activate or deactivate headers using this GraphQL API.

This should be done for both group and instance level streaming

Implementation plan

• This has already been implemented but it disabled waiting on the API update
• Remove disabled property from checkbox on line 587 of ee/app/assets/javascripts/audit_events/components/stream/stream_destination_editor.vue
• Confirm all is working correctly
• Update tests and docs