[r2dbc] Support for one-to-one and one-to-many relationships

Since spring-data-r2dbc is now part of spring-data-relational, I wanted to reopen a much-discussed topic.

Quote from the original issue:

"Spring Data JDBC supports one-to-one and one-to-many (either as set, list or map) relationships:
https://docs.spring.io/spring-data/relational/reference/jdbc/mapping.html#jdbc.entity-persistence.types
It would be really useful to have this feature with R2DBC as well."

Being able to map "simple" relations would make working with r2dbc much easier. Many of our developers do not switch to Spring's reactive stack due to the lack of the ability to represent relations and prefer to stick with the MVC variant, where they can make use of JPA and Hibernate.