This commit introduces a new `dataset_view` parameter to the `get_dataset`
method in the BigQuery client. This allows you to specify the level of
detail (METADATA, ACL, FULL) returned when fetching a dataset.

The `DatasetView` enum has been added to `enums.py`.

Unit tests have been added to verify:
- Correct query parameter (`view`) formation for each enum value.
- Correct behavior when `dataset_view` is None.
- AttributeError is raised for invalid `dataset_view` types.