Add support for DataType polymorphism to the RDB serialization

[pahjbo]

As DataTypes are mapped to columns in RDBs in general there needs to be support for allowing the possibility that DataTypes can be polymorphic - Hibernate has introduced a support for discriminator based embeddable inheritance in version 6.6 that does allow to support polymorphic DataTypes in Java generated code. The 0.6 series of gradle plugins will use this support. This is currently being developed on head from commit e9a7775

[pahjbo]

There are some bugs in hibernate - notably https://hibernate.atlassian.net/browse/HHH-19680 that currently make this unsuitable for production