CDK-472: Add Datasets.URIBuilder

Adds an API for programatic building of Dataset/View URIs.  Also adds
public Dataset.getUri() method.
Because the `FileSystemDataset` is used for both FS and Hive datasets, I'd like to keep this logic in the `DatasetRepository` and pass the URI in when the dataset is constructed. That avoids needing a subclass just for Hive.