Make the Client parameter optional for requests

Make the Client parameter optional when creating auth token, customer or
order requests, and create a new Client instance within the abstract
request if one isn't passed in.

src/Glassboxx/Request/CustomerRequest.php

@@ -34,7 +34,7 @@ final class CustomerRequest extends AbstractRequest implements CustomerRequestIn
 
         $response = $this->client->request(
             'POST',
-            self::BASE_URL . self::ENDPOINT,
+            self::ENDPOINT,
             [
                 'auth_bearer' => $this->authToken,
                 'headers' => [