Implement `ping-pong` for WebSocket server (#782)

* ws-server: Implement `ping-ping`

Signed-off-by: Alexandru Vasile <[email protected]>

* ws-server: Set builder's ping_interval

Signed-off-by: Alexandru Vasile <[email protected]>

* ws-server: Handle just `ping` frames

Signed-off-by: Alexandru Vasile <[email protected]>

* ws-server: Simplify `select`

Signed-off-by: Alexandru Vasile <[email protected]>

* Use `futures_util::select` instead of `select!` macro

Signed-off-by: Alexandru Vasile <[email protected]>

* ws-server: Avoid pinning the delay

Signed-off-by: Alexandru Vasile <[email protected]>

* ws-server: Log when a `Pong` frame is received

Signed-off-by: Alexandru Vasile <[email protected]>

* ws-server: Use tokio for submitting pings

Signed-off-by: Alexandru Vasile <[email protected]>