Create GitHub Action for verifying correct formatting

Since the Debezium project automatically applies code formatting and import sorting, there's much less discussion around code stylistics, as that's handled uniformely. Still every now and then there are PR build failures as the formatter wasn't applied, typically when not running a build locally before.

To make this kind of error better visible, let's add another PR validation step which runs the formatter verification, allowing to easily identify formatting errors from test failures. We could also automate the formatting excecution on CI, but there are reservations that this might be confusing because a merged PR might differ from its submitted state.