Fix type hint for date properties within the Models

[mgoniwiecha]

The Properties $createdAt, $updatedAt and $indexedAt of the model Product (and all other Models that have those fields) are declared as instance of Timestamp but the process never creates those instances. Instead a string is provided like described in the property (for example: Date string of creation, formatted according to RFC 3339.).

Please set the type hint to string or convert that values to an instance of Timestamp.