Access token support

[apvd]

Background

In addition JSON service account credentials, it is sometimes advantageous to using short-lived OAuth2 access tokens to retrieve packages from the artifact registry, for example, in a docker build where writing long-lived service account keys could risk being exported in the image.

Changes

• Adds a configuration file parameter Access-Token-ENV which specifies an environment variable, such as CLOUDSDK_AUTH_ACCESS_TOKEN, that holds an access token.
• After checking for JSON service account credentials and service account emails for the metadata service, we check for an access token environment variable and create the appropriate static token source.

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