Add Checkstyle for Automated Code Style Enforcement

[tzolov]

Add Checkstyle to the project to enforce consistent code formatting and style across the codebase, preventing the need for manual formatting PRs like #392.

[crazybolillo]

I think Spotless plugin would be a better option, what do you think? Besides enforcing, it also auto-formats code. I have had to work with checkstyle in the past and the dev experience feels a bit clunky.

The project can be re-formatted all at once and have that commit in .git-blame-ignore-revs. Another approach also supported by Spotless is to do it in a gradual fashion and enforce formatting only on files that have been modified past a certain tag.

I can help out with a PR. The project needs to chose a formatting style first. I personally like the prettier java format (maintained by JHipster).