`--impersonate` flag that allows CLI to run as a given user

[GavinMendelGleason]

Currently we have no way of running the CLI as a given user. Since access to the store is required to run the CLI it makes no sense to do a full authentication, and yet it would still be useful to run under a lower privilege regime in many circumstances.

Add an --impersonate USERNAME flag to CLI commands to allow authentication to use USERNAMEs authority.