Support relevant subset of JPA annotations

[pellse]

If the POJOs returned from the different method calls to be assembled declare JPA annotation e.g. @Id, @OneToOne, etc., it would be nice to honor those, this would allow to simplify the Assembler API invocation as it wouldn't be mandatory to supply an id extractor function:

So the following:

@Data
@AllArgsConstructor
public class Transaction {
    private final Customer customer;
    private final BillingInfo billingInfo;
    private final List<OrderItem> orderItems;
}

Flux<Transaction> transactionFlux = FluxAssembler.of(this::getCustomers, Customer::getCustomerId)
    .assemble(
        oneToOne(this::getBillingInfoForCustomers, BillingInfo::getCustomerId),
        oneToManyAsList(this::getAllOrdersForCustomers, OrderItem::getCustomerId),
        Transaction::new);

could instead be declared as:

Flux<Transaction> transactionFlux = FluxAssembler.of(this::getCustomers)
    .assemble(
        oneToOne(this::getBillingInfoForCustomers),
        oneToManyAsList(this::getAllOrdersForCustomers,
        Transaction::new);

The idea of providing an id extractor function being actually to avoid to have to support annotations and make the framework a lot more flexible by being able to join/assemble arbitrary data sources, full support for id extraction functions should still be there, and explicitly providing id extraction functions should override the mapping declared in JPA annotations.