## What changes were proposed in this pull request?
This pr is to add a config to control the maximum number of files as even small files have a non-trivial fixed cost. The current packing can put a lot of small files together which cases straggler tasks.

## How was this patch tested?
I added tests to check if many files get split into partitions in FileSourceStrategySuite.

Author: Takeshi YAMAMURO <linguin.m.s@gmail.com>

Closes #12068 from maropu/SPARK-14259.