These beans were added to support easier supplementing of the schemas to load for validation of metadata. They are lists of Spring resources, and should be classpath-based resources for the parser to resolve them properly.
Reserved name of a bean to supply extension schemas to load
Built-in list of extension schemas that can be extended
To add a custom schema, you would define shibboleth.ExtensionSchemas in global.xml or a similar spot and merge that with the parent bean:
<bean id="shibboleth.ExtensionSchemas" parent="shibboleth.DefaultExtensionSchemas">