VHDS Support

Mostly creating this issue to:

1. Get an understanding of any historical context. I see there is an open PR (https://github.com/envoyproxy/java-control-plane/pull/264) but it's stale.
2. Gauge if there is an interest in adding support to the java control plane

I am currently working on an envoy control plane on top of this repository and looking at what could be potentially improved, and this is at the top of the list.