feat(transport/quic_datagrams): expose send queue capacity
Add function `datagram_send_queue_capacity` to `Connection` and `Http3Client`.
Before sending a WebTransport or MASQUE connect-udp datagram, users can check
how many datagrams can still be queued for sending, thus avoiding datagrams
being dropped due to a full send queue.
Enables https://github.com/mozilla/neqo/issues/2852.