[SPARK-16430][SQL][STREAMING] Add option maxFilesPerTrigger

## What changes were proposed in this pull request?

An option that limits the file stream source to read 1 file at a time enables rate limiting. It has the additional convenience that a static set of files can be used like a stream for testing as this will allows those files to be considered one at a time.

This PR adds option `maxFilesPerTrigger`.

## How was this patch tested?

New unit test

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #14094 from tdas/SPARK-16430.