Can be used to add an identifier to the client from an external system, must be unique
{ "id": "string", "external_id": "string", "name": "string", "is_team": true, "users": [ { … } ] }
{ "id": "string", "url": "string", "client_id": "string", "project_id": "string", "signature_secret_key": "string", "event": "contact:create", "created_at": "2019-08-24T14:15:22Z" }
{ "id": "string", "url": "string", "client_id": "string", "project_id": "string", "event": "contact:create", "created_at": "2019-08-24T14:15:22Z" }