Add string as input of of()

[niccokunzmann]

This is a small but nice feature: We expect an icalendar.Calendar object as the first argument to of().
Please allow bytes and string as an input too, parsing them with icalendar.Calendar.from_ical().

To close this:

• add the parameter types
• parse the parameter with icalendar.Calendar.from_ical()
• create a changelog entry
• test your code with bytes
• test your code with string

This would make answers like these way shorter: https://stackoverflow.com/a/79111753/1320237