Add support of relations / associations for DTOs

[lowwa132]

Feature Request

Q	A
New Feature	yes
RFC	no
BC Break	no

Summary

Following this issue #8471, it's recommended to use DTOs instead of partial objects.
Today it's not possible to use DTOs outside of normalized flatten objects (#8471 (comment), #8471 (comment)).

The support of associations with DTOs would provide a full and nice alternative to partials.