Add "response_mode" "form_post" to AbstractOauth2Action

[Hanashi]

The class AbstractOauth2Action is currently only designed for the default response_mode query. But there are OAuth2 providers which require response_mode form_post (e.g. Sign in with Apple). Therefore I ask to change this more dynamically, so that the response_mode form_post can be used as well.

[TimWolla]

Wanted to look into Twitter first, as that is a standalone controller that is reasonably similar. While doing so, I came across cuyz/valinor#358, so putting this on hold for now.