Add checkstyle plugin and config

Looks like the last few commits were still not using the newly added code configuration. To catch such things in the future, I guess makes sense to add the checkstyle plugin as we do in the mailbox.

I added the configuration in a first commit, the CI then fails, as expected because of the violations: https://code.briarproject.org/briar/onionwrapper/-/jobs/28365

After reformatting using AS, the CI succeeds again.