webtransport: use the resource manager to control flow control window increases

[marten-seemann]

Moving from marten-seemann/go-libp2p-webtransport#8.

---

We should use the resource manager to control flow control window increases (similar to

go-libp2p/p2p/transport/quic/conn.go

Lines 48 to 50 in d8c4f16

	func (c *conn) allowWindowIncrease(size uint64) bool {
	return c.scope.ReserveMemory(int(size), network.ReservationPriorityMedium) == nil
	}

).

[marten-seemann]

This is not trivial. The quic-go callback gives us the quic.Connection object. However, in our WebTransport implementation we keep track of webtransport.Sessions.

In general, WebTransport allows the use of multiple WebTransport sessions on a single QUIC connection, although we outlaw this in our spec. I'm having a hard time coming up with a good use case for that in libp2p.

That would still leave us with the problem that we somehow need to be able to correlate the WebTransport session with the QUIC connection. An API change in webtransport-go might be needed.