[spring framework 7] add support for indexing functional bean registrations via BeanRegistrar

[martinlippert]

There is a new way of registering beans in a functional way coming in Spring Framework 7.0, first to appear in M3:
spring-projects/spring-framework#18353 (comment)

We should add support for taking those bean definitions into account when indexing beans.

[martinlippert]

This is almost complete. The remaining piece is to change the indexer for the bean registrar to not create a bean element for the registrar itself anymore, but a specific bean registrar one instead, so that the bean registrar is not regarded as a regular bean by the rest of the tooling.