Programmatic annotation presence registry (allowing for index integration)

[philwebb]

Now that #22420 we have a central location that we can use to investigate if an annotation index can be used to limit the number of methods we need to search.

[jhoeller]

Alternatively, we could also just provide a registration SPI for annotation presence as originally sketched... and leave concrete index evaluation to the application's bootstrap code, so that Boot could evaluate any index format it wants on startup, simply populating the registration SPI with component class + annotation types found on it. If this is the path we'll take, let's narrow this ticket accordingly.

For 5.2 M2, we should make up our mind on this. The registration SPI prototype I had for AnnotationUtils.isCandidateClass worked pretty well. However, we should put it onto its own SPI class so that it can be conveniently delegated to from both AnnotationUtils and AnnotationsScanner.