Add a simple QuicTransport server for testing and demo purposes.

The server currently has two modes, echo and discard.  I've changed the integration test to use the simple server instead of a session with a mock visitor.

This CL also adds some of the missing APIs and fixes some of the bugs I've found while doing this.

gfe-relnote: n/a (code not used)
PiperOrigin-RevId: 278456752
Change-Id: Idf9aa654aa0d66673f300f2f5425f0716d6c3e14
diff --git a/quic/quic_transport/quic_transport_server_session.cc b/quic/quic_transport/quic_transport_server_session.cc
index fe4aec1..7f00acd 100644
--- a/quic/quic_transport/quic_transport_server_session.cc
+++ b/quic/quic_transport/quic_transport_server_session.cc
@@ -66,6 +66,7 @@
   auto stream = std::make_unique<QuicTransportStream>(id, this, this);
   QuicTransportStream* stream_ptr = stream.get();
   ActivateStream(std::move(stream));
+  OnIncomingDataStream(stream_ptr);
   return stream_ptr;
 }