Support response_mode=form_post for Apple adapter

[r2d4]

When requesting scopes with the Apple provider -- there's some bugs with the current adapter:

• You must set response_mode=form_post on the authorization call
• Accept a POST to /callback and parse the code and state from form data
• Name/email are only returned the first time the user goes through the flow, they are added as part of the /callback formdata with the "user" key with the stringified JSON payload

{
    "email": "...",
    "name": {
        "firstName": "...",
        "lastName": "..."
    }
}

"form_post" response mode is actually part of the spec, so it could be implemented with the base oauth2 adapter, although I'm not sure how many other providers would use this. https://openid.net/specs/oauth-v2-form-post-response-mode-1_0.html

Additionally, the user info could be added to the raw tokenset object. Otherwise I'm not sure how the base OAuth2 adapter API should look (do other providers do things like this?)