Refactor streaming audit headers

This change is prep work for [issues 366350][0]

We want to emit audit events whenever the custom streaming event
headers are creating/updated/destroyed

We have decided to refactor the existing logic into [service
classes][1], and then add then add the actual audit events using
[this method][2].

I put the services under `ee/app/services` because the custom http
header feature is [only available in EE gitlab][3]

This first commit is **only** a refactor, so the existing specs have
not been touched and the actual audit event work is not included in
this commit

[0]:#366350
[1]:https://docs.gitlab.com/ee/development/reusing_abstractions.html#service-classes
[2]:https://docs.gitlab.com/ee/development/audit_event_guide/#using-standard-method-call-to-record-single-event
[3]:https://docs.gitlab.com/ee/development/ee_features.html#ee-only-features