Blocker Description
Each worker process's REST API should have an endpoint that can list available connector classes. This can use the same Reflections code as we used in KAFKA-2422 to find matching connector classes based on a short name. This is useful both for debugging and for any systems that want to work with different connect clusters and be able to tell which clusters support which connectors.
We may need a new top-level resource to support this. We have /connectors already, but that refers to instantiated connectors that have been named. In contrast, this resource would refer to the connector classes (uninstantiated). We might be able to use the same resource to, e.g., lookup config info in KAFKA-3315 (which occurs before connector instantiation).