Add "synthetic" entry to DatasetType

[bact]

Propose to add "synthetic" entry to represent synthetic data in the DatasetType vocabulary.

A synthetic data is artificial data that is generated using algorithms, rather than being collected from real-world sources.
It can be used to supplement or replace real-world data in various applications, such as machine learning, testing and validation, and privacy and security.

Having an ability to specify that a dataset is synthetic will help avoiding it being treated as a real-world collected dataset.

@rgopikrishnan91 @bennetkl