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)

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[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.