Expose more `quinn-udp` UdpState properties

[Icelk]

In the implementation of quinn-udp (I'm focusing on UNIX/Linux), the following properties/methods on UdpState are used:

• state.max_gso_segments.store
• state.set_sendmsg_einval()
• state.sendmsg_einval()

They are ~required~ recommended when implementing one's own AsyncUdpSocket. Exposing them would enable more correct external implementations.

I can make a PR if you'd like :)

[djc]

What's your use case?

[Icelk]

Implementing AsyncUdpSocket for tokio-uring.

[djc]

Makes sense, happy to take a look at a PR!