Avoid creating ExecutionPlan for registered tables in Ballista

[rdettai]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
When using the sql() method on the BallistaContext, all the tables registered on the context are transformed into execution plans. We should find a way to avoid that, because scanning datasources can be an expensive operation.

Describe the solution you'd like
I would like a way to register the tables on the Datafusion context without transforming them to ExecutionPlans.

Additional context
#1012 makes the scan operation async. This propagated to the BallistaContext.sql(), which was not expected.