The stream hijackers only need to be able to associate the stream with
the underlying QUIC connection. They are not supposed to call any
functions on the quic.Connection. As such, the better API is to just
pass them a unique identifier.
The http.Request.Body can be type-asserted to a http3.Settingser. The
Settings method on this interface blocks until the client's SETTINGS
frame has been received.
