Endpoint stats interface

[ryleung-solana]

In Anza, we need to get more insight into the resources being used by quinn in servicing quic handshakes. This PR seeks to add an initial interface to the Endpoint struct to allow for getting more detailed stats than just the open connections, and some initial data relevant to the handshakes. On a side note to the Quinn devs, if there is a good way to support getting the current number of open handshakes, we would be interested in know it. Currently, it seems that the Endpoint is not signaled when a Connecting future finishes, and to add such an event would require some refactoring, not to mention handling an extra event, locking, etc.