Standardise on int|WP_User input to the "for user" functions

[dd32]

Currently the various API-level functions accept two main inputs: ID or WP_User, and assumes that if not provided the function should operate on the current user.

Accepts WP_User:

• Two_Factor_Core::get_enabled_providers_for_user()
• Two_Factor_Core::get_available_providers_for_user()

Accepts ID:

• Two_Factor_Core::get_primary_provider_for_user()
• Two_Factor_Core::is_user_using_two_factor()

When a WP_User is passed in place of an ID, or an ID in place of a WP_User, the functions operate on the current user, which could lead to unexpected responses.

This PR standardises the above 4 functions to:

• Accept int|WP_User
• Only default to the current user when no parameters are passed / null is passed (The default value for the function param)
• Uses the same user validation logic via a helper method, rather than duplicating logic.

For example, currently:

wp_set_current_user( 1 );
$incorrect = Two_Factor_Core::get_enabled_providers_for_user( 2 );
$correct = Two_Factor_Core::get_enabled_providers_for_user( get_user_by( 'id', 2 ) );

// $incorrect is for user_id 1, despite 2 being passed in.
// $correct is correct, because a WP_User was passed

After this PR, both $incorrect and $correct in the above would contain the details for user 2.

I'll note that is_user_api_login_enabled() has not been touched, as the $user_id is not used for any logic within the function, and the onus is on the filtering functions to check the value of $user_id passed.