* config: add RemoveRequestHeaders
Currently, we have "set_request_headers" config, which reflects envoy
route.Route.RequestHeadersToAdd. This commit add new config
"remove_request_headers", which reflects envoy RequestHeadersToRemove.
This is also a preparation for future PRs to implement disable user
identity in request headers feature.
* integration: add test for remove_request_headers
* docs: add documentation/changelog for remove_request_headers