This commit adds auto-configuration support for Spring Data R2DBC. If a
`ConnectionFactory` and Spring Data are available, scanning of reactive
repositories is enabled.

This commit also adds a starter to bring R2DBC and the necessary Spring
Data libraries.

See gh-19988
Co-authored-by: Mark Paluch <mpaluch@pivotal.io>
Co-authored-by: Oliver Drotbohm <odrotbohm@pivotal.io>