Get agreed att mtu from connection

[pperanich]

It would be very useful to be able to get the agreed upon att mtu from a connection after the handshake has occured. I think this could take the form of adding a new ConnectionEvent, or by making the Connection::get_att_mtu method public outside of the crate. Open to other implementation ideas as well!

[lulf]

Thanks for raising the issue, I agree. I think exposing the Connection::get_att_mtu() is the way to go, since it's something you want to check outside the place where you're processing the events.