Add a plan to a product

curl --request POST \
  --url https://api.fungies.io/v0/products/{productId}/plans/add \
  --header 'Content-Type: application/json' \
  --header 'x-fngs-public-key: <api-key>' \
  --header 'x-fngs-secret-key: <api-key>' \
  --data '{
  "name": "<string>",
  "description": "",
  "features": [
    {
      "value": "<string>"
    }
  ],
  "cover": "<string>",
  "coverBlurHash": "<string>",
  "coverVideo": "<string>",
  "externalId": "<string>"
}'

{
  "status": "<string>",
  "data": {
    "plan": {
      "object": "variant",
      "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
      "internalId": null,
      "name": "",
      "description": "",
      "order": null
    }
  }
}