[flutter_local_notifications] Add custom repeat interval - periodicallyShowWithDuration method

[mateuszluczak1996]

Add an option to set custom time for periodically showed notifications.

The problem that I have encountered on iOS devices is that repeating interval must be set to 60 seconds minimum, but I don't think that is the problem for real use case scenarios.

Sample usage:

    await flutterLocalNotificationsPlugin.periodicallyShowWithDuration(
      1,
      'repeating period title',
      'repeating period body',
      const Duration(minutes: 5),
      androidScheduleMode: AndroidScheduleMode.exactAllowWhileIdle,
    );

[MaikuB]

Thanks for the PR. Will need time to give this one a good review compared to some other ones

[MaikuB]

In the interest of merging the PR, I've made the changes and will have this merged in soon :)

[yfujiki]

Great addition!!

Only thing I want further is to specify initial time (currently it defaults to current time). But i guess it is limited by iOS side capability.

[pandey407]

Looks great!! About what @yfujiki said, can the offset datetime (say something like the zonedSchedule) be used in tandem with the periodic duration? It might be able to solve multiple scheduling scenarios.