feat: allow the date picker dialog to be opened programmatically (closes #1477)

[geronimol]

Connection with issue(s)

Close #1477

Solution description

Making FormBuilderDateTimePickerState public allows calling the showPicker method programatically. We can access it like so:

final dateTimePickerKey = GlobalKey<FormBuilderDateTimePickerState>();
...
FormBuilderDateTimePicker(key: dateTimePickerKey),
...
dateTimePickerKey.currentState?.showPicker();

Screenshots or Videos

To Do

• [x] Read contributing guide
• [x] Check the original issue to confirm it is fully satisfied
• [x] Add solution description to help guide reviewers
• [x] Add unit test to verify new or fixed behaviour
• [x] If apply, add documentation to code properties and package readme

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.88%. Comparing base (483f02f) to head (76f0a02). Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1478   +/-   ##
=======================================
  Coverage   89.88%   89.88%           
=======================================
  Files          21       21           
  Lines         840      840           
=======================================
  Hits          755      755           
  Misses         85       85           

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[deandreamatias]

Thanks for PR!

This is a sustancial change. I think that need to think more about the consequences of this, as the ones I explain here https://github.com/flutter-form-builder-ecosystem/flutter_form_builder/issues/1132#issuecomment-1295897916

[geronimol]

@deandreamatias I wasn't sure about this either, but I saw the Flutter team is using this behavior in other widgets, such as PaginatedDataTable. You can see they left the PaginatedDataTableState as public, so you can access the pageTo method to dynamically change the page.