Introduce Aggregate Root, Entity, ValueObject, DomainEvent abstracts

[hantsy]

In Eric's DDD, a Repository is tightly tied with Aggregate Root, and used to maintain the lifecycle of a Aggregate Root. And a Aggregate Root could be consist of Entity, Value Objects, and can handle domain events. Have a look at jmolecules project: https://github.com/xmolecules/jmolecules/tree/main/jmolecules-ddd/src/main/java/org/jmolecules/ddd/types

In a JPA, it is easy to maintain the lifecycle like relation between PurchaseOrder and OrderItems, but switching to Jdbc(assume there is a data-jdbc module), it should apple changes to the whole Aggregate when calling a repository save/update etc.