Get a specific user

curl --request GET \
  --url https://api.lunary.ai/v1/external-users/{id}

{
  "id": "<string>",
  "createdAt": "2023-11-07T05:31:56Z",
  "externalId": "<string>",
  "lastSeen": "2023-11-07T05:31:56Z",
  "props": {},
  "cost": 123
}