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.

tests/Glassboxx/Request/CustomerRequestTest.php

@@ -31,8 +31,7 @@ final class CustomerRequestTest extends TestCase
             ->method('request')
             ->with(
                 'POST',
-                CustomerRequest::BASE_URL
-                .CustomerRequest::ENDPOINT,
+                CustomerRequest::ENDPOINT,
                 [
                     'auth_bearer' => $authTokenRequest->getToken(),
                     'headers' => [