Class customization missing from manifest schema

[shuchenliu]

Currently, value under class key in manifest's dumper section is accessed by the loader.py module to decide if a customized dumper class should be used, as shown below

# biothings/hub/dataplugin/loaders/loader.py

klass = dumper_section.get("class")
dumper_configuration = getattr(self, "_dict_for_%s" % scheme)(dumper_urls)

# Add disabled flag to dumper configuration with default value False
dumper_configuration["DISABLED"] = dumper_section.get("disabled", False)

if klass:
    dumper_class = get_class_from_classpath(klass)
    dumper_configuration["BASE_CLASSES"] = klass
else:
    dumper_class = self.dumper_registry.get(scheme)
    dumper_configuration["BASE_CLASSES"] = "biothings.hub.dataload.dumper.%s" % dumper_class.__name__

This extensibility is not reflected in the current schema for manifest.json file, which would cause validation error if class key is present in its dumper section