[spring framework 7] support bean registrars that are defined in libraries

[martinlippert]

As described here:
spring-projects/spring-framework#21497 (comment)

We might arrive at the point where spring projects provide pre-defined bean registrars and users can basically opt-in by defining and importing a concrete specialization of the pre-defined registrar.

For the indexing that we do inside of the language server, this opens up a whole new area: looking into bean definitions from JAR files on the classpath instead of the source files of the projects only (as we do at the moment).

So we need a lot of groundwork for this to have a general mechanism in place that integrates spring index information from JAR files into the index that the language server creates from the source files (see #1524). Once we have that, we can work on taking bean registrars from libraries into account.