Add option to provide token via CLI

[adhilto]

Prerequisites

• This issue has an informative and human-readable title.

💡 Summary

Add option to allow users to directly provide a token via the CLI instead of a path to a credentials file.

Motivation and context

Will be useful for automation.

Implementation notes

Example of authentication via a token string:

    def __init__(self, access_token: str, svc_account_email: str = None):
        self._svc_account_email = svc_account_email
        self._token = Credentials(token=access_token, scopes=self._scopes)
        ...

Acceptance criteria

• Ability to pass token via CLI arg is added

[jacdavi]

To add more context, this will allow running ScubaGoggles from a GCP environment without the need to create a service account key file.

For example, GogglesConnect plans to run in a GCP Cloud Run environment that is already tied to the service account we will use to authenticate. So we already have access, and would prefer to just pass the access token directly rather than having to create the key file (which is sensitive and requires management).

The code to actually do this in Cloud Run is slightly more complicated than expected because you cannot directly change the subject with the default credentials returned. Instead you need to create a new access token (which requires the service account to have permissions to do that). Here is an example of what that code looks like: https://github.com/GoogleCloudPlatform/professional-services/blob/main/examples/gce-to-adminsdk/main.py#L34

I did confirm that this worked in tandem with the code posted in the body.