Add update_time as property for Bucket

[MikeMoore63]

Thanks for stopping by to let us know something could be better!

PLEASE READ: If you have a support contract with Google, please create an issue in the support console instead of filing on GitHub. This will ensure a timely response.

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

So using python library built an app we now need a feature that requires update_time and it does not exist. Its does in Java and in rest api just missing from python implementation. Work around use discovery api.

Describe the solution you'd like
A clear and concise description of what you want to happen.

update_time property supported on the Bucket class properly.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
Additional context
Add any other context or screenshots about the feature request here.