support printing out the authorization token

[wasaga]

Is your feature request related to a problem? Please describe.

sometimes users need to access routes via the command line.
currently users have option of either minting a service account or implementing a programmatic login.

Describe the solution you'd like

pomerium-cli already implements and caches authorization token. we can add a new command that would output a cached token, or, if its not available, would acquire a new one.

# pomerium-cli auth https://my-route.example.com