[REQ] [Dart2] Allow specifying Locale or DateFormat

Is your feature request related to a problem? Please describe.

currently, the api.dart file generates the dateformatter as : final _dateFormatter = DateFormat('yyyy-MM-dd');

This is causing an issue in my app as I need to specify a fixed locale value for the DateFormat() constructor.

This is because the _dateFormatter formats dates according to the current app locale,
which can generate some unicode string dates which the server & the integrations do not accept;
They only accept english formatted dates.

Describe the solution you'd like

the ability to pass the locale to the constructor.

Describe alternatives you've considered

the ability to change the value of _dateFormatter (being var).
this seems to have been a point of discussion earlier, at https://github.com/OpenAPITools/openapi-generator/pull/6389#discussion_r467464348

Additional context

Start with the Dart2-generated api.dart and trace where _dateFormatter is created and used. Confirm how DateFormat currently receives locale information, then define a way for generated clients to use a fixed locale or configurable formatter; done means generated dates no longer depend unexpectedly on the app locale while existing formatting remains intact.