Support for jMolecules' Association type.

[odrotbohm]

We no recognize properties of type org.jmolecules.ddd.types.Association as associations in our PersistentProperty model.

Also, we now register JMolecules' Converter implementations for Association and Identifier in CustomConversions so that they can persisted like their embedded primitive value out of the box.

Fixes #2315.

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[mp911de]

That's merged now.