reverse_tunnel: Implement proactive pings from the downstream_socket_interface

*Title*: reverse_tunnel: Implement proactive pings from the downstream_socket_interface

*Description*:
Currently the downstream_socket_interface does not proactive ping the upstream side using the RPING mechanism -> it only responds to pings this makes the downstream side invisible to reachability.
Worse if the network falls through the downstream never detects it while the upstream thinks the connection is gone. This could have prevented issues like [this](https://github.com/envoyproxy/envoy/pull/46556).
Expectation here is to implement http2 style ping and pong frames and have keepalive duration and timeout before the socket is upgraded to http2 for such detections from both ends.