Ping/Pong implmentation for StreamingMarketDataService

[m1dnightc0der]

Some exchanges implement a ping/pong heartbeats, however I am not sure this is implemented via the streaming services.

It might be useful for upstream applications to issue heartbeats, and then if they fail try to re-initialise the whole connection and was wondering if getPing should be implemented in the StreamingMarketDataSerivice.

An example exchange that implements pings

https://docs.ftx.com/#websocket-api