Creates an alert field

curl --request POST \
  --url https://api.rootly.com/v1/alert_fields \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '{
  "data": {
    "type": "alert_fields",
    "attributes": {
      "name": "<string>"
    }
  }
}'

{
  "data": {
    "id": "8b10c804-9ea3-49aa-9790-6b7224a90bd8",
    "type": "alert_fields",
    "attributes": {
      "slug": "custom-field-test",
      "name": "Custom Field Test",
      "kind": "custom",
      "created_at": "2025-09-24T06:57:58.284-07:00",
      "updated_at": "2025-09-24T06:57:58.284-07:00"
    }
  }
}