Consider a generic way of indicating database setup

[philwebb]

See #17539 for background. It would be nice if instead of listing all different migrations we could have an easy way to say "depends on anything that migrates the database"

[snicoll]

We're considering a dedicated event in Spring Framework to get a chance to do the initialization before anyone start using the DataSource, see spring-projects/spring-framework#21362 for more details.

[philwebb]

I thought we might have something lined up. There's probably a Boot issue open as well.

[snicoll]

A few, actually. #13042 and #17528

[wilkinsona]

I've sketched something out for this here. There's a rough description in the javadoc and some thoughts on possible ways in which it could be changed.