Registration

[ArtyomVancyan]

Motivation:

This PR was mainly committed to adding registration support or providing a way of connecting OAuth users with the existing ORM. But it has changed the following behaviors as well:

• Middleware can obtain an optional callback for calling after authentication success.
• Changed some design solutions that were made before.
• At the code level: fixed all typing issues and added missing ones. Also, added missing descriptions to classes.

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• Have you followed the guidelines in our Contributing document?
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Changes to Core Features:

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• Have you written new tests for your core changes, as applicable?
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