BigQuery: Add `exists_ok=True` option to `create_dataset`

[tswast]

It's common to have to set up a dataset for a script, where you only want to create it if it doesn't already exist.

Currently, one has to manually catch the exception and check if err.code == HTTPStatus.CONFLICT: # 409

Instead, it would be ideal if there was an exists_ok option, similar to how the Path.mkdir function can catch and ignore the FileExistsError for you.