## What changes were proposed in this pull request?
Add a maven plugin and wrapper script to use scalafmt to format files that differ from git master.
Intention is for contributors to be able to use this to automate fixing code style, not to include it in build pipeline yet.
If this PR is accepted, I'd make a different PR to update the code style section of https://spark.apache.org/contributing.html to mention the script
## How was this patch tested?
Manually tested by modifying a few files and running ./dev/scalafmt then checking that ./dev/scalastyle still passed.
Closes#23148 from koeninger/scalafmt.
Authored-by: cody koeninger <cody@koeninger.org>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
