BigQuery: implement defaultEncryptionConfiguration on datasets

[shollyman]

This feature request is to ensure the client library supports default encryption settings on
BigQuery dataset resources.

On a dataset, defaultEncryptionConfiguration (an instance of EncryptionConfiguration) can
be used to indicate a default CMEK key that will apply to all new tables or ML models created
within the dataset. Existing models are not affected, and this can be overridden when a
(table or model) is created by specifying the settings explicitly.

For libraries that expose underlying discovery types directly, this request may be a no-op.

For more information, please contact @shollyman