Use spotless plugin for formating

[rnett]

Uses the spotless plugin for formatting. Will check formatting in the initialize phase (so its before codegen & native builds), and auto-format can be ran with mvn spotless:apply. We can use it for the Kotlin API as well once diffplug/spotless#142 is fixed.

I have it set to ratchet from master (i.e. only check changed files), do we want to do a big "apply formatting everywhere" PR (it's ~500 files)?

[karllessard]

I'm not convinced about reformatting source code with Maven, any reason why simply using Google Java Style formatting isn't enough?

[rnett]

The XML differs a bit from the actual plugin, at least for me (things like line length and single line comments). Plus, it's easy to forget and have a bunch of improperly formatted files end up in the repo, like we have now. This doesn't actually do any formatting, it just checks it as part of the build, like the lint checks.

[karllessard]

@rnett I should have labeled that PR with CI build before merging... can you please check the multiple errors we are facing on many platforms from GitHub Actions? I might need
to revert that PR unless you find a quick fix for it, thanks

[rnett]

Ugh, it seems to be that --add-exports isn't recognized for JDK 8, but is required for 16. I'll remove and ignore that file for now.

[rnett]

Do you or @Craigacp know if there's a way to have a version-dependent .mvn/jvm.config file?

[rnett]

See #324.