Allow to configure arbitrary headers passed in http calls.

[dmajere]

For sentry.io production infrastructure we plan to have some envoy route matching based on additional header.
Allow to configure arbitrary headers passed to upstreams.

[dmajere]

Used host_header as example, added #4713

[Dav1dde]

@dmajere Can envoy not just itself add that header? Istio can do it, so why do we need that in the application?

[dmajere]

No, this header is used by envoy to pick correct upstream.

[Dav1dde]

I don't understand this use case, if you want a host header, you should just configure the upstream URL correctly, especially the host header is the header which is controlled by the HTTP client framework, messing with that is not a good idea.

[dmajere]

Here is the full explanation of a problem and solution we aim to
https://www.notion.so/sentry/US-Frontend-routing-1eb8b10e4b5d808fb561f335c15ad423?pvs=4