Add support for `BeanRegistrar` implementing `RuntimeHintsRegistrar`

[sdeleuze]

Would be useful for native-compliant classpath checks described in #21497 for example, but likely also for other use cases.

[sdeleuze]

I am not sure we can easily support BeanRegistrar implementing RuntimeHintsRegistrar with the GenericApplicationContext#register use case, so I will decline this issue, as there are alternative ways to declare RuntimeHintsRegistrar (META-INF/spring/aot.factories or @ImportRuntimeHints).