Improve formatting of logical plans containing subquery expressions

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
When we implemented subqueries in the logical plan, we did not update the display logic, and the plans are hard to read because the indentation is not correct.

Describe the solution you'd like
Fix the formatting. Here is a proposal:

Projection: #employee_csv.id
  Filter: #employee_csv.state IN (<subquery>)
    Subquery:
      TableScan: employee_csv projection=[state]
    TableScan: employee_csv projection=[id, state]

Describe alternatives you've considered
None

Additional context
None