Feature: Make use of spotless maven plugin

[spotlesscoder]

Overview of the feature request

For Java, I have made good experience with spotless. I saw in issue #6379 there was already a discussion about it.

@PierreBesson: I read your comment from December 12, 2017 in that discussion. The build does not necessarily have to fail when the formatting is not correct because spotless can apply the correct formatting, too.

Motivation for or Use Case

Ensure clean source code files by using a formatted for all source files.
Introducing automated code formatting as early as possible in a project has some great benefits:

• Better readability of source code because only necessary code parts are in the file (no unused imports, spotless can remove them). Also, the code will look more equal across different parts that might be written by different developers
• Less whitespace / reformatting noise when someone touches existing code. When automated formatting is used, the formatting should always look the same no matter who edited the code. This keeps the git diff clean because no unnecessary whitespace changes etc. should be present there when formatting is done automatically.

Related issues or PR

#6379

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