Command to list available variants

[JakeDawkins]

Avery had the great idea of adding a command to list out the variants available to a graph. Right now, if a user runs a command with an invalid variant name, we just point them to studio. It'd be nice if we had the ability to ask them to run something like rover variant list graph-name to get a list of variants.

If we can't find a better place to put this command other than a new top-level namespace, we could also expand the variant commands to include things like variant delete, but that's for another issue :)

[lrlna]

Should include did you mean this one? for when a wrong variant was provided.