HTTP/1.1 Header casing

[bcollard]

Description

Please add support for header casing propagation.

Some legacy apps (or language frameworks) don't support HTTP 2 headers with lower casing.
For instance, some client apps create HTTP 1.1 headers with a specific case that is expected as-is by the backend app.

We need this on the Kuma Gateway: https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_conn_man/header_casing#config-http-conn-man-header-casing

This option would help some customers adopt the mesh.

In particular, we need this on the upstream cluster HTTP protocol options, with the stateful formatter and the PreserveCaseFormatterConfig (empty config block {} is a good start)

[slonka]

Triage: makes sense

[slonka]

Triage: need to look at envoyproxy/envoy#15619