feat: change on_accept in ProtocolHandler to better deal with 0rtt connections

[rklaehn]

To handle 0rtt connections you have to do the entire handling of the connection in on_accepting, since it returns a Connection, but you do get a IncomingZeroRttConnection.

In light of this, 1. the on_accepting name is not really that fitting anymore. Also, there is no way to signal that you have successfully completed except obtaining a Connection from the IncomingZeroRttConnection via handshake_completed after the connection is already closed.

So we should either rename on_accepting and make it return an Option, or rethink the IncomingZeroRttConnection newtype.

[rklaehn]

Link to discussion: https://discord.com/channels/949724860232392765/1433060357936779274

[rklaehn]

Maybe the solution would be to not copy the entire connection API into (Incoming/Outgoing)ZeroRttConnection but to make (Incoming/Outgoing)ZeroRttConnection a struct containing just the future and the connection?

I just checked, and the API of (Incoming/Outgoing)ZeroRttConnection is really just a complete copy of Connection, and there isn't really significant different behaviour for any of the methods.

[rklaehn]

Trying something here: #3619