[DEPRECATED] Creates an incident custom field selection

curl --request POST \
  --url https://api.rootly.com/v1/incidents/{incident_id}/custom_field_selections \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '{
  "data": {
    "type": "incident_custom_field_selections",
    "attributes": {
      "custom_field_id": 123,
      "value": "<string>",
      "selected_option_ids": [
        123
      ]
    }
  }
}'

{
  "data": {
    "id": "2",
    "type": "incident_custom_field_selections",
    "attributes": {
      "custom_field_id": 173,
      "incident_id": "9d9bde4e-8742-46c8-8e8c-e19015395ae8",
      "value": "Test custom field",
      "selected_option_ids": []
    }
  }
}