Possibility to register wordpress authenticate filter.

[gassan]

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Changes proposed in this Pull Request:

This PR is a replacement for #130

Added authenticate_filter to allow login via a default wordpress Login form. The user is not forwarded to Provider Server.

If we are owner of both instances (a WP site and User Provider Server), we can allow the WP instance to know a user credentials - so user can login direct via Login Form.

The 'new' validate($response_token) function is not new. the functions body is moved from authentication_request_callback() and adapted to reuse with new authenticate_filter() function.

Closes # .

How to test the changes in this Pull Request:

1. Install some provider that allows grant_type password (we are using Keycloak) and select checkbox 'Register Authenticate Filter'. Try to login with username and password.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changelog entry

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[timnolte]

Please don't alter the Pull Request Template.

[gassan]

Should I review this PR. Are there any chances of merging for it?