Create SubscriptionItemFeature

curl --request POST \
  --url https://app.flowglad.com/api/v1/subscription-item-features \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '{
  "subscriptionItemFeature": {
    "livemode": true,
    "subscriptionItemId": "<string>",
    "featureId": "<string>",
    "productFeatureId": "<string>",
    "type": "toggle",
    "amount": {},
    "usageMeterId": {},
    "renewalFrequency": {},
    "expiredAt": "2023-11-07T05:31:56Z"
  }
}'

{
  "subscriptionItemFeature": {
    "id": "<string>",
    "createdAt": "2023-11-07T05:31:56Z",
    "updatedAt": "2023-11-07T05:31:56Z",
    "createdByCommit": "<string>",
    "updatedByCommit": "<string>",
    "livemode": true,
    "subscriptionItemId": "<string>",
    "featureId": "<string>",
    "productFeatureId": "<string>",
    "type": "toggle",
    "amount": {},
    "usageMeterId": {},
    "renewalFrequency": {},
    "expiredAt": "2023-11-07T05:31:56Z"
  }
}