Fetch form templates by optional parent category(ies), or all form templates if omitted

Example response

[
  {
    "form_type": "SH04",
    "form_name": "Notify a sale or transfer of treasury shares",
    "form_categories": [
      {
        "id": "SCOT",
        "name": "Scottish jurisdiction"
      }
    ],
    "fee": "10.00"
  }
]