Support multiple Kafka clusters

A nice to have feature would be for the proxy to communicate with multiple Kafka clusters. This would allow a deployment of the websocket proxy to be the "single point of entry" towards Kafka infrastructure inside a secured network, reducing both complexity and potential attack surface.

Thoughts on implementation details:

• Configuration will have to be provided for > 1 Kafka cluster.
• The proxy will require a "primary" cluster for its internal topics, etc.
• Clusters need to be identified with an unique ID inside the proxy config.
• Should have an endpoint to retrieve information on which clusters are available through the proxy.
• API will require an additional argument so the clients can specify which cluster to target.
• It might be a good idea to add an endpoint that can list topics, with info on which target a topic is located on. However, this requires some thought with regards to ACLs and security considerations.

This above list will evolve as the thought process matures...