Enable getting and setting custom QUIC transport parameters.

This is primarily for WebTransport (https://tools.ietf.org/html/draft-vvv-webtransport-quic-00#section-3.2), but may be useful for other purposes in the future.

gfe-relnote: n/a (v99 only)
PiperOrigin-RevId: 268811939
Change-Id: Icf4ba1657ffbca367d72110be86047052bd5dcdf
diff --git a/quic/core/quic_config.cc b/quic/core/quic_config.cc
index 6400b0e..e48d21a 100644
--- a/quic/core/quic_config.cc
+++ b/quic/core/quic_config.cc
@@ -978,6 +978,7 @@
   initial_round_trip_time_us_.ToHandshakeMessage(
       params->google_quic_params.get());
   connection_options_.ToHandshakeMessage(params->google_quic_params.get());
+  params->custom_parameters = custom_transport_parameters_to_send_;
 
   return true;
 }
@@ -1085,6 +1086,8 @@
     }
   }
 
+  received_custom_transport_parameters_ = params.custom_parameters;
+
   *error_details = "";
   return QUIC_NO_ERROR;
 }