Improve support for @Nested and @NestedTestConfiguration in various test framework contracts

[wilkinsona]

This is related to #31456.

It's unclear to me exactly where support for @Nested and @NestedTestConfiguration fits into the test framework's various contracts. Clarifying that would, I hope, make it easier to understand which APIs and utility methods support @Nested and which require implementers of those contracts to roll their own support. Hopefully the situations where implementers need to roll their own support can then be reduced and, ideally, eliminated entirely.

One specific example is AnnotationConfigContextLoaderUtils. I'm rather confused about the purpose and general utility of AnnotationConfigContextLoaderUtils if it doesn't support @Nested and @NestedTestConfiguration semantics.

Perhaps this would be better handled as two issues? One to make some additions to the javadoc to describe the current situation and steer implementers in the right direction and a second to reduce the cases where implementers need to do anything at all.

[snicoll]

@sbrannen is this a documentation or are they any API change scheduled? An issue can't be both. If API changes are expected, I'd argue we probably need to reconsider the milestone.