[dart][dio] add dateLibrary options ( core, timemachine )

Created by: amondnet

This pr adds dateLibrary option and simple test to dart-dio generator. dateLibrary has two options.

• core : dart:core, (default), not handled properly date, date-time format; https://github.com/OpenAPITools/openapi-generator/issues/2696 I will make a new PR on this.
• timemachine: time_machine

If dateLibrary is timemachine,

input

openapi: 3.0.0
info:
  contact:
    email: amond@amond.net
  description: test
  title: test
  version: 1.0.0
paths:
  /test:
    get:
      responses:
        200:
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/DateContainer'
          description: OK

components:
    schemas:
      DateContainer:
        type: object
        properties:
          date:
            format: date
            type: string

output

import 'package:time_machine/time_machine.dart';
import 'package:built_value/built_value.dart';
import 'package:built_value/serializer.dart';

part 'date_container.g.dart';

abstract class DateContainer
    implements Built<DateContainer, DateContainerBuilder> {
  @nullable
  @BuiltValueField(wireName: 'date')
  LocalDate get date;

  // Boilerplate code needed to wire-up generated code
  DateContainer._();

  factory DateContainer([updates(DateContainerBuilder b)]) = _$DateContainer;
  static Serializer<DateContainer> get serializer => _$dateContainerSerializer;
}

@ircecho @swipesight @jaumard @nickmeinhold

--

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