Support for impersonation

[morgante]

Ideally this tool would support service account impersonation so a user can act "as" a service account without needing to download a JSON key.

[lindsayismith]

This tool either uses application default credentials (meaning it looks to see if you're on a VM and uses that SA if so), or it delegates to 'gcloud auth print-access-token'.

I think what you're asking is if it could run be configured to run

'gcloud auth print-access-token --impersonate-service-account ...'

to get that token, is that correct?

[yihanzhen]

maybe we just do this: #55