[WON'T FIX] To improve performance, support the use of data pinned to caches for entities that are intended to be reference data. For these, infer the ImmutableFacet.

That is, allow the developer to add the DN extension:

@Extension(vendorName="datanucleus", key="cache-pin", value="true")

as a way to pin any reference data into memory.

If present, we should infer that the entity is immutable (ImmutableFacet).