[xDS] add auto_host_rewrite to human-readable form of route config (#38119)

Missed this in #37087.

Closes #38119

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/38119 from markdroth:xds_route_to_string_includes_auto_host_rewrite 64e75359a5
PiperOrigin-RevId: 696537855

src/core/xds/grpc/xds_route_config.cc

@@ -199,6 +199,7 @@ std::string XdsRouteConfigResource::Route::RouteAction::ToString() const {
   if (max_stream_duration.has_value()) {
     contents.push_back(max_stream_duration->ToString());
   }
+  if (auto_host_rewrite) contents.push_back("auto_host_rewrite=true");
   return absl::StrCat("{", absl::StrJoin(contents, ", "), "}");
 }