This commit introduces the `UpdateMode` enum and integrates it into the
`update_dataset` method in the BigQuery client.

The `UpdateMode` enum allows you to specify which parts of a dataset
should be updated (metadata, ACL, or full update).

The following changes were made:
- Defined the `UpdateMode` enum in `google/cloud/bigquery/enums.py`
  with values: `UPDATE_MODE_UNSPECIFIED`, `UPDATE_METADATA`,
  `UPDATE_ACL`, and `UPDATE_FULL`.
- Modified the `update_dataset` method in
  `google/cloud/bigquery/client.py` to accept an optional
  `update_mode` parameter. This parameter is added to the query
  parameters if provided.
- Added unit tests in `tests/unit/test_client.py` to verify the
  correct handling of the `update_mode` parameter, including
  testing all enum values and the default case where it's not
  provided.