[SPARK-20773][SQL] ParquetWriteSupport.writeFields is quadratic in number of fields

Fix quadratic List indexing in ParquetWriteSupport.

I noticed this function while profiling some code with today. It showed up as a significant factor in a table with twenty columns; with hundreds of columns, it could dominate any other function call.

## What changes were proposed in this pull request?

The writeFields method iterates from 0 until number of fields, indexing into rootFieldWriters for each element. rootFieldWriters is a List, so indexing is a linear operation. The complexity of the writeFields method is thus quadratic in the number of fields.

Solution: explicitly convert rootFieldWriters to Array (implicitly converted to WrappedArray) for constant-time indexing.

## How was this patch tested?

This is a one-line change for performance reasons.

Author: tpoterba <tpoterba@broadinstitute.org>
Author: Tim Poterba <tpoterba@gmail.com>

Closes #18005 from tpoterba/tpoterba-patch-1.