log signal_strengt into HomeAssistant

[pointhi]

Is your feature request related to a problem? Please describe.

Based on #2115, as when we can set the entity_category=diagnostic we can also send additional diagnostic data, like the RSSI. The signal strenght is already reported via mqtt, but not interpreted by home assistant as it is not part of the current auto-discovery part.

Describe the solution you'd like
Add HA auto-discovery for RSSI as diagnostic information.

Example of how this could look like:

[1technophile]

Indeed, could be interesting to add. PR welcome also for this