Extend StatefulHeaderFormatter to allow forwarding HTTP1 reason phrase (#18997)

Signed-off-by: Max Kuznetsov <mkuznetsov@digitalocean.com>

Mirrored from https://github.com/envoyproxy/envoy @ 76a70b40f57bd9a75b50d4783d28dec0e0aa29ae

envoy/extensions/http/header_formatters/preserve_case/v3/preserve_case.proto

@@ -16,4 +16,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // See the :ref:`header casing <config_http_conn_man_header_casing>` configuration guide for more
 // information.
 message PreserveCaseFormatterConfig {
+  // Allows forwarding reason phrase text.
+  // This is off by default, and a standard reason phrase is used for a corresponding HTTP response code.
+  bool forward_reason_phrase = 1;
 }