Support forking of plugin

[klaraward]

Support forking of the plugin to be able to set jvm args needed to run with Java 16+.

After doing so, also remove the args in .mvn/jvm.config and in <argLine> introduced in #125

[Stephan202]

@klaraward will this be an optional feature? I'm asking because our builds have suitable args in .mvn/jvm.config for other reasons anyway, so we'd rather not unnecessarily pay for the overhead of forking the JVM.

[klaraward]

Yes, forking will definitely be optional.

[klaraward]

I plan to work on this the coming days.

[danielnorberg]

Resolved by #140

[norrisjeremy]

Can this be adjusted to always add the --add-exports & --add-opens statements when forkMode is set to always?
This would allow users a way to remove the An illegal reflective access operation has occurred warnings that are seen when using Java < 16.

[danielnorberg]

@norrisjeremy I reopened #106 to continue the discussion on whether and how to eliminate these warnings on Java < 16.