Support setting initial congestion window via BoostIcwHeader proto.

When initial_congestion_window is specified in h3_behavior in response headers, set the initial congestion window on the session's send algorithm (clamped by kMaxInitialCongestionWindow) if not previously set.

Protected by quic_reloadable_flag_quic_boost_icw_set_cwnd.

PiperOrigin-RevId: 982490030

quiche/quic/core/quic_sent_packet_manager.h

diff --git a/quiche/quic/core/quic_sent_packet_manager.h b/quiche/quic/core/quic_sent_packet_manager.h
index b9bbe92..dd6dc64 100644
--- a/quiche/quic/core/quic_sent_packet_manager.h
+++ b/quiche/quic/core/quic_sent_packet_manager.h
@@ -420,6 +420,8 @@
     return send_algorithm_.get();
   }
 
+  SendAlgorithmInterface* GetSendAlgorithm() { return send_algorithm_.get(); }
+
   void ReduceMemoryUsage() {
     unacked_packets_.ReduceMemoryUsage();
     send_algorithm_->ReduceMemoryUsage();