Add support for entity_category in auto discovery

[xZise]

Hi, I'm wondering if it would be possible to add methods to manipulate the "entity_category" of a entity?

To be honest, I'm not sure what are valid combinations (can a button be a "diagnostic" entity?), so maybe it would be sufficent to just have a boolean marking an entity as "non-primary" (see also registry properties) and depending on the type it would set entity_category either to diagnostic or config.

[cccmangus]

#276