Major feature

Adding Home Assistant calendar. So that the next collections for each calendar will show in Garbage Collection Calendar. And there will also be a new calendar created, with the next event.

I am looking forward to your feedback. Does it work? Is it useful? Any suggestions?
Please write in the issue #161.
Thanks!

Considerations/Limitations

In the ConfigFlow, HA does not allow me to have a common configuration for the garbage_collection platform (because the sensors are not physical devices that are not discovered, I have to add each entity as new integration (since there is no way in HA to manually add new "devices" into one integration - there is no place for common configuration).
Because of that, there is no place to configure the calendar name :(. So it is hard-coded.
The alternative would be to create a calendar for each schedule, that would not work I suppose.