QUIC Key Update support

Handles key updates initiated remotely and also adds a QuicConnection method to initiate a key update, but this method is currently only called in tests.

Protected by FLAGS_quic_reloadable_flag_quic_key_update_supported.

PiperOrigin-RevId: 336385088
Change-Id: If74d032e1d34e5392312f4b619d28c9f93a95265
diff --git a/quic/core/quic_connection_stats.cc b/quic/core/quic_connection_stats.cc
index 191918c..0cc6a73 100644
--- a/quic/core/quic_connection_stats.cc
+++ b/quic/core/quic_connection_stats.cc
@@ -55,6 +55,7 @@
   os << " num_ack_aggregation_epochs: " << s.num_ack_aggregation_epochs;
   os << " sent_legacy_version_encapsulated_packets: "
      << s.sent_legacy_version_encapsulated_packets;
+  os << " key_update_count: " << s.key_update_count;
   os << " }";
 
   return os;