Set default progress_bar option to None

This change sets the default value of `bigframes.options.display.progress_bar` to `None`.
This means progress bars will be disabled by default, instead of automatically detecting the environment ("auto").
Users can still enable them by setting the option to "auto", "terminal", or "notebook".

This change also adds a unit test to verify the default value and the behavior of the option context manager.

Author: google-labs-jules[bot]