Make ballista an optional dependency of datafusion-cli

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
We are trying to release a new version of datafusion (#1587 ) and we found that since we are not doing a parallel release of ballista, we can't release datafusion-cli because it depends on ballista -- see #1587 (comment)

Describe the solution you'd like
I would like datafusion-cli crate to have ballista as an optional dependency so it can be published to crates.io prior to the ballista release

Describe alternatives you've considered

1. Don't release datafusion-cli
2. Break it into its own repo in datafusion contrib

Additional context
Add any other context or screenshots about the feature request here.

[alamb]

I am going to take a shot at this to see how easy/hard it might be