Load aggregates that have a single one-to-m relationship with a single select

[schauder]

This is the first step of implement single-select-querying for Spring Data JDBC.

It will only work for aggregates that have

• at most a single one-to-m relationship.
• no embedded entities.
• no nested entities.

Single Query Loading will be used for the relevant methods of CrudRepository

It should be implemented in a way that allows integration in Spring Data R2DBC