Use provider methods instead of model for 2FA

[makstech]

Description:

Current 2FA validation ignores custom authentication providers. When there's a provider defined that extends Illuminate\Auth\EloquentUserProvider with a custom query in newModelQuery method, it is ignored and instead, user search is done by calling on models, so it bypasses newModelQuery method.

Steps To Reproduce:

1. Create a provider that extends Illuminate\Auth\EloquentUserProvider.
2. Overwrite newModelQuery method and add to query something that would certainly not return a user, like

    protected function newModelQuery($model = null)
    {
        return parent::newModelQuery($model)
            ->where('id', '=', -1);
    }

3. Try to log in with a gate that has the previously created provider.
4. \Laravel\Fortify\Actions\RedirectIfTwoFactorAuthenticatable::validateCredentials will still find the user and will try to validate credentials.

[driesvints]

This was attempted before by me here #189 and then reverted again. I'm not sure why we reverted that in the end.

Taylor has also stated that he thinks it's no problem that Fortify requires Eloquent: #180 (comment)

[mpyw]

@driesvints I guess the real reverted reason here: #189 (comment)
Actually this PR #318 does not have any problems, LGTM.