[BUG] [Dart] Documentation for auth uses defaultApiClient.getAuthentication<ApiKeyAuth> but no such methods exists

Description

See: https://github.com/OpenAPITools/openapi-generator/blob/df05e6f4bc6265ba4de801ba306f75dd7278e119/samples/openapi3/client/petstore/dart2/petstore_client_lib/doc/UserApi.md

import 'package:openapi/api.dart';
// TODO Configure API key authorization: api_key
//defaultApiClient.getAuthentication<ApiKeyAuth>('api_key').apiKey = 'YOUR_API_KEY';
// uncomment below to setup prefix (e.g. Bearer) for API key, if needed
//defaultApiClient.getAuthentication<ApiKeyAuth>('api_key').apiKeyPrefix = 'Bearer';

But the ApiClient class has no such method. Here are the available methods on the object:

invokeAPI()
addDefaultHeader()
deserializeAsync()
serializeAsync()
deserialize()
serialize()

There's a .authentication object but the interface only has a single void applyToParams(List<QueryParam> queryParams, Map<String, String> headerParams); function.

Consequently, I don't know what the canonical way to do auth, and specifically Cookie Auth is.

openapi-generator version

commit 559a2618dfe62dc8b86d0eb169bb4d998774a21f, Sunday May 15 2022

Start with the generated sample file samples/openapi3/client/petstore/dart2/petstore_client_lib/doc/UserApi.md and compare its authentication example with the Dart ApiClient methods and authentication interface described in the issue. Trace how Cookie Auth is represented in the generated client, then update the documentation so it uses available APIs and states the canonical authentication setup.