Create category, merchant feedback

Example request

{
  "transaction": {
    "category": {
      "heron_id": "ctg_Hyv4jvQtWZcTH8jVL5sHpt",
      "label": "Insurance"
    },
    "merchant": {
      "heron_id": "mrc_hJcvPCaAgesp9etcNh6H6J",
      "name": "Spotify"
    }
  }
}