Introduce AuthorizationRequestInterface

[lordrhodos]

This PR introduces a new AuthorizationRequestInterface which is implemented by the current AuthorizationRequest. This way implementors are provided with a more flexible way to override certain parts of the authorization grant types than with the concrete class alone.

In my case I was looking into a way to use a decorated authorization request to be able to add the nonce attribute used of an OpenID Connect (OICD) implementation. I understand that this library is only supporting the OAuth2 specs and nothing related to OIDC (see #962) which is totally fine with me.

This PR raises the flexibility of the library without doing any harm 🙂

[lordrhodos]

related to #1003

[Sephster]

Looks good to me @lordrhodos although this is a breaking change so will need to go in version 9.

[lordrhodos]

Thank you @Sephster 👍

[iNviNho]

Looks good :)

[shineability]

@lordrhodos Would you be willing to share how you fixed the nonce issue using the decorator? I solved it in a very hacky way by saving the nonce on the User when authenticating and then retrieve the value from the User again when generating the ID token in the response. I could use a more elegant solution :)