Add a way to tell if an SCMFileSystem.Builder supports an SCMSource class, not just an instance

This has been annoying for me - I've got a thing that only wants to allow you to choose an SCMSource if that SCMSource has an SCMFileSystem.Builder implementation that's valid for it. SCMFileSystem#supports(SCMSource) (which calls Builder#supports(SCMSource) for each Builder extension) would be fine...except that I want to know which SCMSource implementations are valid in a configuration UI, meaning I don't have instances, just descriptors. So it'd be awfully handy to have Builder#supports(Class<? extends SCMSource>) or Builder#supports(SCMSourceDescriptor).