Add ping timer management in WebSocketManager

[lifeimm100]

This update introduces a ping_timer attribute to the _WebSocketManager class, ensuring that the ping timer is properly canceled when closing the websocket connection.

[dextertd]

The user can already control how frequently the websocket pings with the ping_interval arg. What's the benefit of exposing this ping_timer as well?

[kolya5544]

^ should probably be a private field