Add Home Assistant entity_category support

[xannor]

It would be helpful if the entity_category attribute could be added to the entity discovery message for entities that are configuration and/or diagnostic in nature. This would help reduce clutter when the HA entities is re-exported through other services (I.E. Home Bridge) as most will ignore categorized entities by default.

The following integrations support entity_category so it is not just limited to sensor:
binary_sensor
switch
select
and many others...

[tsightler]

Feel free to submit a PR and I will review it.