[BUG][dart-dio] add package:built_collection/built_collection.dart to imports

[ahmednfwela]

Description

API end points that require sending BuiltList<String> will fail since this is missing from the imports

package:built_collection/built_collection.dart

Ref:

openapi-generator/modules/openapi-generator/src/main/resources/dart-dio/api.mustache

Lines 1 to 5 in c322084

	import 'dart:async';
	import 'dart:convert';
	import 'package:dio/dio.dart';
	import 'package:built_value/serializer.dart';

openapi-generator version

V5.0.0

[kuhnroyal]

Yes I have just had this happen to me with BuiltMap, this is my bad. I will add a fix shortly.