Add addon to subscription

curl --request POST \
  --url https://api.cloud.flexprice.io/v1/subscriptions/addon \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '{
  "addon_id": "<string>",
  "end_date": "<string>",
  "metadata": {},
  "start_date": "<string>"
}'

{
  "addon_id": "<string>",
  "addon_status": "active",
  "cancellation_reason": "<string>",
  "cancelled_at": "<string>",
  "created_at": "<string>",
  "created_by": "<string>",
  "end_date": "<string>",
  "entity_id": "<string>",
  "entity_type": "subscription",
  "environment_id": "<string>",
  "id": "<string>",
  "metadata": {},
  "start_date": "<string>",
  "status": "published",
  "tenant_id": "<string>",
  "updated_at": "<string>",
  "updated_by": "<string>"
}