Which issue does this PR close?

Towards #3554

Rationale for this change

Separate the actual writing logic, so it can be extended/modified later, rather than be tangled within the shuffle writer.

What changes are included in this PR?

1. Created a writers module
2. Moved BufBatchWriter and PartitionWriter in said module
3. Exposed specific functionality required by other code, rather than have everything available as before.
4. Removed the to_df_err function, as DatafusionError already has a #[from] clause for all these errors, added a map_err() on the outer function calls so we add a backtrace and know the error is from the shuffle(the to_df_err function essentially just added a "error in shuffle write" prefix to each error)

How are these changes tested?

The regular tests should cover these changes.