Connect Rest Extension Plugin issue with Class Loader

KAFKA-6776 implementation has 2 issues with the class loader.

1. If a RestExtension plugin is available in the classpath, then the plugin gets associated with all the plugin class loaders. This is happening because the Manifest resource is searched in the parent class loader as well by default. 
2. The class name associated with the plugin class loader is incorrect. It should be the implementation class instead of the interface.