oauth: add sign-in with apple support (fixes #110)

[AlexCuse]

Implements an oauth provider for sign-in with apple. Requires some additional flexibility in oauth credentials / providers:

• Include a map of additional data in credentials.
• Allow providers to override secret behavior - configured secret in apple credentials is a private key used to sign a secret calculated at runtime, in this case a JWT that includes additional data as claims.
• Allow providers to accept returns as HTTP POST instead of GET.
• Allow providers to add additional oauth options to authorization request.

[AlexCuse]

One thing I noticed so far trying to test is that Apple doesn't like including the scopes in the auth request unless response_mode=form_post is specified. Should be pretty straightforward but guess we'll need a post handler for oauth return 😢

[AlexCuse]

One thing I noticed so far trying to test is that Apple doesn't like including the scopes in the auth request unless response_mode=form_post is specified. Should be pretty straightforward but guess we'll need a post handler for oauth return 😢

We just needed to redirect POSTs to the existing handler and allow providers to override the method for routing.