Add an IPv6 flow label field to QuicPacketWriterParams so that packet
writers can set this field when writing packets. Currently nothing sets
this field.

Modify the raw packet composer to set this field when writing packets.

PiperOrigin-RevId: 676054135

quiche/quic/core/quic_packet_writer.h

diff --git a/quiche/quic/core/quic_packet_writer.h b/quiche/quic/core/quic_packet_writer.h
index c2b92c6..fccdea9 100644
--- a/quiche/quic/core/quic_packet_writer.h
+++ b/quiche/quic/core/quic_packet_writer.h
@@ -43,6 +43,8 @@
   bool allow_burst = false;
   // ECN codepoint to use when sending this packet.
   QuicEcnCodepoint ecn_codepoint = ECN_NOT_ECT;
+  // IPv6 flow label in host byte order. Only the low 20 bits will be used.
+  uint32_t flow_label = 0;
 };
 
 // An interface between writers and the entity managing the