feat(Sessions): Add remember me feature via a filter

[menno-ll]

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Changes proposed in this Pull Request:

Closes #512 .

This change allows the user to, using a filter, login users via the rememberme option in WordPress.
The wordpress login cookies will then be set for 14 days by default instead of two days as a session.

I originally wanted to use the token expiration date to do so, however this doesn't seem possible as you cannot pass an expiration date to the wp_set_auth_cookie function. So instead I chose to implement the remember me feature as used by WordPress itself.

What could maybe be possible is to dynamically add a filter for auth_cookie_expiration, so the WP user cookie expiration can be set to the same value. This filter needs to be added temporarily, and then removed.
However this is kind of hacky and i think it would be better to submit that as a separate PR after this one is approved.

How to test the changes in this Pull Request:

1. In your theme, add add_filter( 'openid-connect-generic-remember-me', '__return_true' ); to the functions.php file.
2. Login using the openid connect plugin
3. Open the development tools of your browser, and look at the cookie expiration. It should not be a session cookie, and it should be valid for 2 weeks.
   

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

Added the openid-connect-generic-remember-me filter to allow theme makers to login users using the remember me option.

[menno-ll]

I've added another PR #514 that actually sets the WP user cookie expiration time to the token expiration time of the IDP.

[menno-ll]

This PR unit test does not succeed because of a connection error, not because the test itself is failing.
I however don't have the ability to re-run the unit test without making a change, which seems kind of useless.
If you could please restart it, that would be great. Thanks!