* Centralize OAuth2.0 Credential logic
Adds a Credentials class that centralizes and handles most existing
logic related to OAuth2.0 credentials, including generation, storage,
file locking, and attribute retrieval. This is a step towards
minimizing the duplicated code that handles credentials in various
methods. The goal is to eventually get to a point where there are 2
credential entry points: `auth.get_admin_credentials()` and
`auth.get_credentials_for_user(user)`. Then, we can slowly move toward
using impersonated credentials for all operations and scrap the need
for user consented credentials all together.
* Skip test_delete_removes_lock_file when testing on Windows
