GH-2119 Migrated the AggregateReference to relational module

[mipo256]

Closes #2119

[schauder]

I approve in principle.

But if we move AggregateReference into relational, R2DBC should properly support it.

Also, the DCO check is failing.

[mp911de]

This is a breaking change without deprecation and that doesn't work for us.

Before considering such a change, we should investigate much wider, Spring Data REST has a org.springframework.data.rest.core.AggregateReference and have a general idea how such a change fits generally into Spring Data. We should also investigate a design where AggregateReference is part of Spring Data commons with retrofitting JDBC and REST.

Note that investigate doesn't mean start implementing all the things and submitting various PR's but rather sketch a design and capture architectural decisions, constraints and effects onto today's users of the API.

[schauder]

I'm rejecting the PR, since as Mark pointed out we need more design considerations on this one.