[DDC-3304] Add support for embeddables in entity generator

Currently the entity generator does not respect embeddables and therefore generates wrong entity classes.
This PR adds support for generating embeddables and embedded properties in entities.
It also introduces a new option to decide whether embeddable classes should be mutable or immutable. Mutable embeddables (default) are generated like entities (getters and setters but no constructor). Immutable embeddables however do not provide setters but instead have a constructor that takes all defined properties as arguments.