Enforce the upcoming formatting guidelines by checking them during the CI build

[tsaglam]

If we implement #495, we should also try to enforce these guidelines. A separate CI-workflow should check each PR and fail if the guidelines are not applied correctly. Developers should have an easy means of auto-formatting i.e. a dedicated mvn command to call.

When starting to enforce these guidelines, we require a dedicated commit that formats the whole project once. The timing for that commit needs to be selected carefully, to avoid excessive conflicts with branches and forks.

For this, we could use Spotless. It can use formatter configuration files but also enforce additional rules such as re-ordering pom files. If spotless is part of the maven configuration, the formatting can be applied with mvn spotless:apply.
A CI workflow can be based on the one from JPlag. However, we need to figure out the compatibility with Xtend and the Reaction files.

In the long run, we could also employ Sonarqube.

[tsaglam]

For Xtend we cannot easily apply the formatter, but at least it is easy to:

• enforce indent with space
• enforce no trailing whitespaces
• check/enforce via custom regex rules

[tsaglam]

Spotless allows to enforce formatting gradually, which would avoid a single monolithic formatting commit:
https://github.com/diffplug/spotless/tree/main/plugin-gradle#how-can-i-enforce-formatting-gradually-aka-ratchet

[TomWerm]

As vitruv-tools/Maven-Build-Parent#36 reverted the Spotless configuration changes, we should discuss whether or not we still plan to implement this PR @vitruv-tools/maintainers .