Allow user to replace auto-configured Spring Data JDBC beans with their own

[snicoll]

Spring Data JDBC does not have a JdbcDataConfiguration. Rather we only have an auto-configuration for the repositories support that brings an extension of org.springframework.data.jdbc.repository.config.AbstractJdbcConfiguration. I've noticed this when looking at the ManagedTypes support new for replacing classpath scanning in AOT scenario.

[wilkinsona]

Having discussed this with @mp911de, the main problem is that Boot sub-classing AbstractJdbcConfiguration prevents users from providing their own replacement for any of the beans that it defines. We can overcome this by overriding the inherited @Bean methods and annotating them with @ConditionalOnMissingBean as EnableWebMvcConfiguration already does in a few places for some of MVC's beans. This approach will make things more flexible for users while avoiding having to duplicate the moderately complex logic in the @Bean methods of AbstractJdbcConfiguration.