List custom field definitions

Example response

{
  "success": true,
  "data": [
    {
      "identifier": "industry",
      "key": "industry",
      "name": "Industry",
      "entity_type": "organisation",
      "type": "text",
      "description": "The primary industry of the company.",
      "group": "custom",
      "is_visible": true,
      "source": "manual"
    }
  ]
}