Add DataModelProperties

The current Properties object only handle String => String.

Using a new object, extending Properties that can handle String => Serializable will avoid to serialize to JSON and then deserialize the properties before setting them on a DocumentModel, for instance using the UpdateDocument operation.

This is useful when calling AutomationService directly from the server where we don't want to serialize / deserialize before calling DocumentModel#setPropertyValue.

The DataModelProperties handles only DataModels.