Add classic loader module and the ability to easily switch implementations

[philwebb]

#37668 is a risky endeavour for a point release so we should make it easy for folks to remain on the older loader code if they wish to do so.

[xtermi2]

We have class loading issues since spring boot 3.3.3 (see #42087) and I wantet to try the classic classloader. Where can I find the documentation to easily switch implementations @philwebb?

[bclozel]

@xtermi2 does this help? https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-3.2-Release-Notes#nested-jar-support