Migrate from Checkstyle to Spotless

[Scott-Guest]

This PR migrates all of our formatting checks from the Checkstyle plugin to the Spotless plugin.

As a result, mvn package (or mvn spotless:apply or mvn process-sources) automatically fixes any formatting violations, e.g., inserting copyright headers for you.

The diff is large, but the only non-automated code changes are in pom.xml and src/main/config/checkstyle-*.xml

All the behavior is the exact same except:

• Previously, copyright headers could match the regex Copyright \(c\) (Runtime Verification, Inc|K Team). All Rights Reserved., but now we only accept the Runtime Verification, Inc version due to limitations of Spotless. All existing headers were converted automatically
  • If there is an issue with this legally, I believe there is a mechanism to avoid changing the copyright of any existing files, but all new files would still require Runtime Verification, Inc
• We more broadly enforce spaces over tabs, unix line endings, newlines at the end of each file, and no trailing whitespace. Previously, this was only enforced for K and Java files.

[radumereuta]

lgtm now, although very large.
Maybe someone else can have a look. If not, you can merge in a few days.

[Baltoli]

Looks good to me also (once my laptop had loaded the diff page!)