Add code_verifier and code_challenge to Authorization class

[molcay]

Hi,

I was trying to implement Authentication in my felt app with my web application which uses the django-oauth-toolkit for OAuth 2.0. I tried setting up the OAuth application with Authorization flow.
As you can see in the django-oauth-toolkit documentation, it needs the following extra parameters:

• code_challenge
• code_challenge_method
• code_verifier

The underlying oauthlib.oauth2 already has these parameters in the following methods:

• prepare_request_uri method has code_challenge and code_challenge_method
• prepare_request_body method has code_verifier

With this PR, now we pass these values to the underlying library to use the Authorization flow.

[CLAassistant]

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