Migrate AggregateReference to the relational package

[mipo256]

The AggregateReference is now a part of JDBC module solely, which is a bit strange from my perspective, since the AggregateReference makes sense it R2DBC module as well when moduling the relationships.

Maybe we want to migrate it to the relational module?

[mipo256]

The problem become even more obvious, when dealing with #2113. When dealing with composite foreign keys via AggregateReference, it turned out that we need to, among other things, make the MappingRelationalConverter aware of the existence of an AggregateReference, which seems reasonable, but the MappingRelationalConverter is located in relational module, whereas AggregateReference is located under jdbc module

[schauder]

I agree that it would be nice to move AggregateReference into relational. But in order to not confuse people more than necessary, we should properly support it in R2DBC.

For #2113 it would be sufficient to just fix it in the JdbcMappingRelationalConverter.

[schauder]

And we should do it in a backward compatible way. I.e. create a new AggregateReference, let the old one extend the new one and deprecate it.

[mipo256]

That makes sense, @schauder, thank you.

I'll focus now on #2113, since this bug seems to be more important. I think we will be able to get by the AggregateRefernce limitation. Once I'll complete the #2113, I'll get back here and extend this PR to account for the problems you've pointed out.