[main-lts] Exposed the serializable-model when generating client models.

[github-actions]

Backport: #907

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Issue: #869

Exposed the possibility to generate models that implement the java.io.Serilizable-interface. I changed the property from the proposed serializableModel to serializable-model since I thought it harmonized better with the naming of the other configurations.

I wrote an new test module for this, maybe it was a bit over engineered, but I didn't find any existing suitable test to put this in, so I did a new module instead.

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