Devin D'Angelo

### Is your feature request related to a problem or challenge? The partition_by COPY option is multivalued, e.g.: ```sql COPY table to file.parquet (partition_by 'a,b,c') ``` This is handled currently...

## Which issue does this PR close? related to #11182 ## Rationale for this change Splitting `FileFormat` implementations out of datafusion-core will help ensure datafusion remains modular and extensible, as...