Handshake resource exhaustion limit

[rday]

Problem:
There is a limit provider which customers can use to set limits in the system. We should have a way to set a limit that indicates handshakes should be deferred. This would be used as an indicator that QUIC should send Retry Tokens to clients.

Possible Solutions:
Update the limit provider to allow customers to set a max_concurrent_handshake limit.

[rday]

The Limits provider is not the right place for this specific functionality. The current "Limits" will refer to limits on a Connection (max streams, flow control, etc). This issue introduces a way to manage resources at the Endpoint level.

When accepting new connections, an Endpoint can make a decision about how to proceed. This decision is returned as an enum to the library. This enum will tell the library what to do with the connection.

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Outcome {
    Allow,
    Retry { delay: Duration },
    Drop,
    Close { delay: Duration },
}

#[non_exhaustive]
pub struct ConnectionInfo<'a> {
    current_handshakes: usize,
    source_address: &'a SocketAddress,
}

pub trait Format {
    fn on_connection_attempt(&mut self, info: &ConnectionInfo) -> Outcome;
}