Add status to gateway device #537
Description
In order to refactor the Gateway status sensor in ramses_cc from direct gwy querying (not async) to async state, the Gateway Entity should expose a status (recent messages via the device = True) as a property.
Currently, the integration (client) tries to access gateway.engine.this_message and compare that message's timestamp to a cutoff datetime. This no longer works in 0.55.6
Copy pattern from battery_state