DataSourceInitializer is not reusable

[snicoll]

In its current form, there is no easy way to initialize a secondary DataSource the same way Spring Boot does via auto-configuration. For one org.springframework.boot.autoconfigure.jdbc.DataSourceInitializer is package private (and has the same name as a feature in the core framework). It also has this DataSourceInitializedEvent that may (or may not) be relevant for a secondary DataSource.

The basic features take a DataSource and a DataSourceProperties. Perhaps we should have a public contract that can be reused and use an internal extension for our specific use cases?

This prevents to complete the documentation update (see #7652)

[snicoll]

We've discussed that one and are not keen to act on it at the moment.