Add support for Embedded Objects

Describe the solution you'd like

@Entity()
class User {
  User(this.id, this.address);

 @Id
  final int id;

 @Embedded()
  final Address? address;
}

class Address {
 final String? streetAddress1;
 final String? streetAddress2
}

Database schema for User then becomes

id
street_address_1
street_address_2

Describe alternatives you've considered

The alternatives are as mentioned in https://github.com/objectbox/objectbox-java/issues/456

Additional context

This request is similar to the features provided in other persistence libraries

• Room
• Isar

No repository files or tests are named in the issue. Start by reviewing the linked ObjectBox Java issue and the Room and Isar embedded-object references, then locate the Dart entity and schema-generation entry points. Done means the example model is supported and its User schema stores the embedded address fields as street_address_1 and street_address_2.