Add Apple as a provider

[erral]

I have been working on adding Apple as an authentication provider into Authomatic.

To do so I had to introduce 2 changes in the OAuth2 base class:

• Add a method to get the secret for the access token request phase: Apple's OAuth2 implementation requires a specific secret to be sent in the access token request. This secret is a JWT token encoded with a secret that Apple provides when registering the client. That's why I introduce a method to get the credentials, which in base OAuth2 just returns the consumer secret, and in Apple returns the dedicated JWT token. To do so, I introduce PyJWT as a requirement of the package.
• Add a method to get extra params that Apple requires in the access token phase: Apple requires to send the response_mode param, so I add a dummy method that returns nothing in the base OAuth2 class and then the method that returns what Apple requires in its own baseclass.

I don't exactly know whether Apple's implementation is a fully OAuth2 compliant implementation, so I am OK if this PR is completely disregarded. I just wanted to let you know about these changes and discuss them before integrating into Authomatic.