UnityAuth should recognize credential environment variables when run under Jupyterhub

[mcduffie]

Running unity_sds_client under MDPS Jupyterhub instances should not prompt for username/password. The auth information is already available as environment variables. Instead it should use this mechanism:

https://github.com/unity-sds/unity-ads-deployment/blob/84b94314a2a08b8cc9fba89b779d18418cf55a30/ci_cd/activation_example/unity_token.py#L6

This would be a modification to UnityAuth in:
https://github.com/unity-sds/unity-monorepo/blob/main/libs/unity-py/unity_sds_client/unity_session.py