Introduce RuntimeHintsUtils#registerComposableAnnotation

[snicoll]

Working on #28767 and the use case in Spring Boot revealed that we could use an explicit method to register a composable annotation when we know it is going to be the used with attribute override (@AliasFor). This requires not only the regular annotation hint but also a proxy.

The only way to trigger this is to register a composed annotation (i.e. an annotation using that annotation) but if the underlying code is only using the annotation and not of the composed one, that would register an unnecessary hint.