Improve support for configuration metadata clients without classpath access

In our Spring Boot application we have serveral custom configuration properties. As some configuration properties are mantained by domain experts, it's not feasible to give them a text file (properties or yaml) to configure the application.

Therefore we plan to provide a graphical configuration tool which contains uses the Configuration metadata to render certain controls, so users are able to configure the application using a graphical user interface.

The Configuration Metadata Spring Boot provides is a very good starting point, but unfortunately there is certain information missing due to the fact that you don't consider them to be needed in the metadata file. As stated in https://github.com/spring-projects/spring-boot/issues/7562#issuecomment-264687830 by @snicoll the list of enum literals is not included as this can be extracted by the IDE. Unfortunately, in our scenario we don't have an IDE or access to the classpath.

Therefore it would be very helpful to have as many information available in the metadata file as possible to provide the best user experience in our configuration client.

Start with Spring Boot's configuration metadata generation and the discussion linked in issue 7562. Determine which information a classpath-free graphical client needs; done means the generated metadata exposes that information without IDE or classpath access.