Config Converter for Class objects

[dmlloyd]

It should be possible to have Class objects in configuration, as long as the configuration root in question is loaded at run time (either during static init, or rarely, during the main method) and not at build time. For the configuration of build steps, the generic type of the class can be used to generate the appropriate asSubclass call. Classes in configuration would be loaded and cast as the configuration is loaded; missing classes would produce an error. Static init would be the preferred time for this because loading classes in the main method would require them to have previously been registered for reflection.

[dmlloyd]

This one depends on #425 / #770 / etc. in order to provide the config system with a usable class loader during config value conversion.