Add controls around injected headers (#2240)

* Add controls around injected headers

This PR adds controls to which headers should be added to
requests/responses. This solves (most of)
istio/istio#17635, a common feature request.

A prototype is implemented in istio/istio#37215.

Open questions:
* Naming bikeshed
* Mesh config or proxy config or something else? Currently its in proxy
  config
* Is a list of ENUMs the best way to represent this? it does allow a
  strange config like `[REQUEST_ID, REQUEST_ID]` but that can just
rejected

* address comments

* xfcc