Simplify TestRuntimeHintsRegistrar API #29264
Description
The TestRuntimeHintsRegistrar currently combines processing of MergedContextConfiguration and test classes. However, it appears that only spring-test internals have a need for registering hints based on the MergedContextConfiguration. For example, Spring Boot's AOT testing support has not had such a need.
In light of that, we should simplify the TestRuntimeHintsRegistrar API so that it only focuses on test classes, and we should move the hint registration code specific to MergedContextConfiguration to an internal mechanism.