SpringBoot Result from SpringApplication.from(...) may need to provide access to more details in the future

With #35203 we changed SpringApplication.from(...) to return the ApplicationContext but it's possible we may need to expose more items in the future. For example, we might want to provide access to SpringApplication or to other ApplicationContext instances if there's a hierarchy.

Although it's late, I think it's worth introducing a new interface to that we have a natural place to add future methods.