add support for java.time.Duration

[massimosiani]

This is the core changes needed to support java.time.Duration. Because I am not familiar with the JDL tool (my fault, it looks great!), any feedback is very very very welcome. Thank you!

see https://github.com/jhipster/generator-jhipster/issues/9095

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[MathieuAA]

@massimosiani Only one thing left to do is to update the JDL page here

[massimosiani]

Hi @MathieuAA, just to confirm, is it this one?

[MathieuAA]

Perfect!

[murdos]

@MathieuAA : I'm ready to merge the PR on generator-jhipster. Could we merge this one?

[jdubois]

Thanks & congrats @massimosiani for your first PR to jhipster-core !