Provide WP config variable for app secret

[PatelUtkarsh]

• [ ] update readme with required variable name

If we restrict PHP to 7 and above then we can use array define in PHP as it's supported from 7 https://www.php.net/manual/en/function.define.php.

define( 'GOOGLE_LOGIN_CONFIG', [
                 'client_secret' => '',
                 'restrict_domain' => '',
          ] );

[dhaval-parekh]

This has covered in #4 PR : #14