add a way for the user to get current datagram limit

[nanokatze]

Right now there's no way for the user to know how big they can make their datagrams before they're dropped during packing and thus have to size their datagrams to something conservative. Having an API to get current datagram size limit would let some users utilize available packet space better.

[marten-seemann]

API-wise, it would be reasonable to add this to the ConnectionState. The value would be min(peer limit, max frame size according to current MTU / connection ID length). The second part would be: mtu - header size - aead overhead - framing overhead.
This feels a little bit brittle, but we could have an integration test that checks that the value reported actually works, and that value+1 doesn't work.

I'd be happy to review a PR, if you (or anyone else) want to work on this. Please let me know if you have any concerns or different ideas.