refactor: Using public dynamic methods instead of static class constants

[frascuchon]

Motivation:

Some backend implementations use the base methods authorization_url and access_token_url instead of the constant settings.

This PR makes a minimal change to support using method instead of class constants, which can be more flexible in some cases (for example the Auth0OAuth2which use the base domain to compound these urls)

All Submissions:

• [ ] Have you followed the guidelines in our Contributing document?
• [ ] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes to Core Features:

• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you updated the documentation related to the changes you have made?
• [ ] Have you written new tests for your core changes, as applicable?
• [ ] Have you successfully run tests with your changes locally?

[ArtyomVancyan]

@frascuchon, I appreciate your interest and effort in contributing to the project, but as you can see, not all social-core backends implement the authorization_url and access_token_url methods that cause to initiation errors. If you can suggest an update that will consider that case, I would love to see it.

[ArtyomVancyan]

@frascuchon, currently closing the PR because of inactivity. Feel free to reopen in the future when you have other suggestions on this topic.