routeCapsule method
Route a capsule received on the given streamId.
If no session exists for streamId, one is created before forwarding
the capsule.
Implementation
void routeCapsule(int streamId, WebTransportCapsule capsule) {
final session = _sessions.putIfAbsent(
streamId,
() => WebTransportSession(streamId),
);
session.onCapsuleReceived(capsule);
}