[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": {
    "attributes": {
      "custom_field_id": 123,
      "value": "<string>",
      "selected_option_ids": [
        123
      ]
    }
  }
}
'

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