Allow OAuth providers to pass additional options to provider constructor

[tadhgboyle]

A simple change which helps us support things like Keycloak.
As you can see from its readme, it wants some additional config options passed to the constructor (authServerUrl, realm, etc):
https://github.com/stevenmaguire/oauth2-keycloak#authorization-code-flow

This has been tested on a demo keycloak instance thanks to @RediPanda

        NamelessOAuth::getInstance()->registerProvider('keycloak', 'Keycloak Module', [
            'class' => \Stevenmaguire\OAuth2\Client\Provider\Keycloak::class,
            'user_id_name' => 'sub',
            'scope_id_name' => 'profile',
            'icon' => 'fas fa-key',
        ], [
            'authServerUrl'  => 'https://sso.nekonii.xyz',
            'realm' => 'devrealm',
        ]);