Add `base_config()` accessor function for `NdJsonExec`

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do. I am trying to produce visual query plans in Ballista and this exec does not allow me to access the file scan config. The other file scan execs do.

Describe the solution you'd like Add this method - see ParquetExec for example

Describe alternatives you've considered

Additional context

[alamb]

Not sure if this is helpful: https://github.com/apache/arrow-datafusion/blob/81b57948778cfa485d8b3f2c1fe4485f3966e8e4/datafusion/expr/src/logical_plan/plan.rs#L604-L633

https://docs.rs/datafusion/12.0.0/datafusion/logical_plan/enum.LogicalPlan.html#method.display_graphviz