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

**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**

Locate NdJsonExec and compare its interface with ParquetExec, which the issue names as the example. Add the requested base_config() accessor so the file scan configuration can be accessed for visual query plans, then confirm it matches the corresponding file-scan executor behavior.

Written by the indexing model from the issue text.