add Rest Contact Custom Fields List endpoint

src/Infusionsoft/Api/Rest/CustomFieldService.php

@@ -0,0 +1,14 @@
+<?php namespace Infusionsoft\Api\Rest;
+
+use Infusionsoft\Api\Rest\Traits\CannotCreate;
+use Infusionsoft\Api\Rest\Traits\CannotDelete;
+use Infusionsoft\Api\Rest\Traits\CannotSave;
+use Infusionsoft\Api\Rest\Traits\CannotSync;
+
+class CustomFieldService extends RestModel {
+
+    use CannotDelete, CannotSync, CannotSave, CannotCreate;
+
+    public $full_url = 'https://api.infusionsoft.com/crm/rest/v1/contactCustomFields';
+
+}

src/Infusionsoft/Infusionsoft.php

@@ -693,6 +693,13 @@ public function appointments()
         return $this->getRestApi('AppointmentService');
     }
 
+    /**
+     * @return \Infusionsoft\Api\Rest\CustomFieldService
+     */
+    public function customfields()
+    {
+        return $this->getRestApi('CustomFieldService');
+    }
 
     /**
      * @return \Infusionsoft\Api\Rest\TransactionService