[MNG-7354] Refactor MavenCli to ease extensibility

[gnodet]

@michael-o This is a small refactoring, the goal is mainly to make mvnd main class not to rewrite the whole MavenCli.
The consequence for mvnd is in the following commit apache/maven-mvnd@6d2fb40.

[cstamas]

Nice!

[michael-o]

Note that this property will likely go away with #602

[gnodet]

Reading #602, I'm not sure I agree with adding more intelligence in the launch scripts and removing it from the java code. This will make tools embedding maven much more difficult to behave consistently with maven.

[AlexanderAshitkin]

Allowing to ovveride dozens of methods makes MavenCli contract vague and could complicate miantenance. going forward It will be easier to maintain MavenCli with public api/interface extracted. For clients it will be easier as well to implement custom logic by delegation. So far it is unclear how to use and extend this class correctly and safely

[gnodet]

Allowing to ovveride dozens of methods makes MavenCli contract vague and could complicate miantenance. going forward It will be easier to maintain MavenCli with public api/interface extracted. For clients it will be easier as well to implement custom logic by delegation. So far it is unclear how to use and extend this class correctly and safely

The main usage is apache/maven-mvnd@6d2fb40