spring index information for libraries

[martinlippert]

If we want to use and show bean information from classes that are implemented as libraries (e.g. via spring projects that users have on the classpath of their projects), we need to find a way to integrate information about beans (and everything else we have in the spring index) that comes from JAR files instead of source files.

The overall idea here is that the libraries provide this information in JSON format as a metadata file, similar to the metadata files for configuration properties.