datasets: include uri in api output for dvcx

[dberenbaum]

See https://github.com/iterative/dvcx/pull/1321/files#r1547778555. dvc.api.get_dataset() should return something like {"name": "dogs-and-cats", "version": 1, "uri": "ds://dogs-and-cats@v1"} (adding the "uri" field).