Cannot add custom response types to OAuth2Authenticator

[gustavoja]

Xamarin.Auth Issue

Version

• nuget version = 1.7.0
• component version =
• sample

Steps to reproduce

1. Try to create a OAuth2Authenticator and add several custom response types

Platform:

• [] .NET version = all
• [] mono version = all

Expected behaviour

The OAuth2Authenticator should allow custom response types. In my case i need "token openid"

Actual behaviour

Response types are hardcoded only for "token" and "code"
VerifyOAuth2FlowResponseType is midway adapted but is not using VerifyOAuth2FlowResponseType function who should create a list with the custom "response types". At the moment it only returns a null object instead of returning a new list of response types built with the types provided by the user.

IF IT IS A NEW FEATURE REQUEST, INCLUDE THIS PART:

Feature description

There should be an easy way to add custom/extra response types on OAuth2Authenticator. I'm thinking on adding a new flag like "HasExtraResponseTypes" and a property List ExtraResponseTypes to obtain the user provided response types an add it to the "response_types" list.

https://github.com/xamarin/Xamarin.Auth/blob/master/source/Core/Xamarin.Auth.Common.LinkSource/OAuth2Authenticator.cs