Add support for response_mode=form_post

[geoff-va]

Looking to add support for the optional response_mode=form_post.

In this case, after authorization, instead of a redirect to the redirect_uri with the params in the query string, a page is rendered with a form including those variables in it and POSTed (autosubmit via JS) to the redirect url.

Relevant Links:

• https://openid.net/specs/oauth-v2-multiple-response-types-1_0.html#ResponseModes
• https://openid.net/specs/oauth-v2-form-post-response-mode-1_0.html

[alexalligator]

Hi @juanifioren @geoff-va, would love to see this issue & PR merged. Hope this fantastic little library is still active.
Thanks for your work on this project.

[Annelies-H]

Would be great if this could be part of the library. We require a form post response in one of our flows and out now testing it with the code from the PR.