Make timezone database initialization optional

[kightsonsanom]

1. In case the application initializes its own timezone by setting the local timezone as the current device timezone, it should be possible to omit the initialization of the timezone database so that it doesn't default to UTC.
2. Update flutter_native_timezone library to the latest 2.0.0 version.

[thomassth]

Nice, didn't think it would be easy to implement.

How does it behave when upgrading from 3.x or the current 4.x prereleases?

ie. how many lines would people need to fix when upgrading

[kightsonsanom]

No changes will be required, this is an optional parameter with a default value so the call to DeviceCalendarPlugin() will always initialize the timezone database as before

  factory DeviceCalendarPlugin({bool shouldInitTimezone = true}) {
    if (shouldInitTimezone) {
      tz.initializeTimeZones();
    }
    return _instance;
  }