[DEPRECATED] Retrieves an incident custom field selection

curl --request GET \
  --url https://api.rootly.com/v1/incident_custom_field_selections/{id} \
  --header 'Authorization: Bearer <token>'

{
  "data": {
    "id": "<string>",
    "type": "incident_custom_field_selections",
    "attributes": {
      "value": "<string>",
      "selected_option_ids": [
        123
      ],
      "incident_id": "<string>",
      "custom_field_id": 123
    }
  }
}