Create Foreign Keys for references between aggregates.

[schauder]

Similar to #1599 but with respect to references between aggregates, represented by AggregateReference instances.

[gmelon]

Hi @schauder, now that the within-aggregate part (#1599) is implemented, is this still a valid enhancement? If so, I'd like to work on it.

My plan is to extend the approach from #1599 (original PR #1629) to references between aggregates: for columns mapped from AggregateReference properties, resolve the referenced aggregate root's table and identifier column via the mapping context, and generate foreign keys the same way intra-aggregate references are handled. If the referenced entity type is not part of the schema generation scope, I would skip the foreign key rather than fail.

If that direction sounds reasonable, I'll submit a PR with tests.

[schauder]

That sounds good.