feat: add optional padding parameter to calendar

[Snorlax8]

Hi! We wanted to have an option to give the calendar some padding. What we were specifically looking for, was a way for the calendar to have a bottom margin, but we decided it would be more useful to have a flexible parameter that just worked with whatever EdgeInsets it was given. The use case for this feature can vary depending on the UI. In our case, we had a bottom sheet that, when popped up, almost completely covered the last month of the calendar.

The change we made allows for the calendar to have an additional space, like this:

[casvanluijtelaar]

please merge the latest master to resolve analyzer issues