Wrap cli in our own 'icav2' binary to prevent needing to 'source' at the start of scripts

The source wrapper is convenient for environment variables but is really only needed for the `projects enter` and `tenants enter` commands. 

Other plugins could be called in a nested 'icav2' binary which would be added to path as part of the .*rc invocation. 

This would mean that users wouldn't need to re-source the *rc at the start of the script to use the cli plugins