Enable -U to override context user (in new cobra based CLI)

[stuartpa]

Tracking PR feedback:

Where do we hook up the -U switch on the CLI as an override of the user name of the current context?

Need to think about the wider issue of how to enable all the backcompat sqlcmd switches to be passed into the new sqlcmd subcommands, e.g. how many switches from old sqlcmd do we need to make available to sqlcmd query