Minimal product definition, used by apps declaring products for sale.
| id | Unity IAP product ID. Potentially independent of store IDs. |
| storeSpecificId | The ID this product has on a store. |
| type | The product type. |
| ProductDefinition | Create a ProductDefinition with different Store-independent ID and Store-specific ID. |
| Equals | Compares id properties. Requires obj be a ProductDefinition. |
| GetHashCode | Gets the application-domain-specific hash code of id. |