Move HTTP/3 Datagram Flow ID Allocation Service to QuicSpdySession

This CL is part of a chain of refactors of the CONNECT-UDP code aimed at allowing code reuse between MASQUE and WebTransport.

The modified code is not used in production.

PiperOrigin-RevId: 359618731
Change-Id: Ia97597e31a28ff602ee3910fb9b5667b9ac82924
diff --git a/quic/core/quic_constants.h b/quic/core/quic_constants.h
index 2de827f..fcfec62 100644
--- a/quic/core/quic_constants.h
+++ b/quic/core/quic_constants.h
@@ -291,6 +291,13 @@
 QUIC_EXPORT_PRIVATE extern const char* const kEPIDGoogleFrontEnd;
 QUIC_EXPORT_PRIVATE extern const char* const kEPIDGoogleFrontEnd0;
 
+// HTTP/3 Datagrams.
+enum : QuicDatagramFlowId {
+  kFirstDatagramFlowIdClient = 0,
+  kFirstDatagramFlowIdServer = 1,
+  kDatagramFlowIdIncrement = 2,
+};
+
 }  // namespace quic
 
 #endif  // QUICHE_QUIC_CORE_QUIC_CONSTANTS_H_