Storing token_type in extra_data field when using OAuth 2.0

[clintonb]

This value is useful for authentication servers (AS) that may provide multiple types of tokens. Most servers use the Bearer token type; however, we at @edx are transitioning to using JWT. The ability to make the switch without having to change our backend would be ideal. Storing the token type means we can avoid hardcoding the value and let the access token response determine which scheme is used for future requests made using the issued token.

[clintonb]

@omab please review. FYI @bderusha @nasthagiri @jcdyer

[clintonb]

@omab any feedback?

[omab]

@clintonb thanks for this PR, I've merged it but ran PEP8 on it first.

[clintonb]

Thanks @omab!