* 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
* xds: use plain functions, add unit tests for control plane routes
* xds: add test for grpc routes
* xds: add test for pomerium http routes
* xds: add test for policy routes
* xds: use plain functions
* xds: test get all routeable domains
* xds: add build downstream tls context test
* more tests
* test for client cert
* more tests
