#336 use the with_subject method

[jackred8]

Description

If you have delegated domain-wide access to the service account and you want to impersonate a user account, use the with_subject method of an existing ServiceAccountCredentials object. This is to resolve #336.

Fixes # (issue)

Has it been tested?

• [x] Development testing done
• [x] Unit or integration test implemented

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have performed a peer-reviewed with team member(s)
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] Any dependent changes have been merged and published in downstream modules