Refactor ParquetExec::try_from_files in preparation for making it parallel

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
The logic in `ParquetExec::try_from_files` is not written in a way that makes it easy to parallize.

**Describe the solution you'd like**
Refactor logic to make it easier to apply threading in a separate PR.

**Describe alternatives you've considered**
N/A

**Additional context**
N/A
