Multiple VHDS subscriptions cannot be multiplexed over ADS

When multiple VHDS subscriptions are multiplexed over a single connection, the server doesn't have a way to associate a given VHDS subscription with a route config: there's no identifying information in the initial request.

Normally, when a subscription first starts it passes resource name(s) that it subscribes to with the first request. VHDS operates on a collection of vhost resources, however, and we currently don't have a way to identify a particular collection -- when a VHDS subscription starts, its initial request contains an empty resource name list.

ping @htuch.

This issue affects the fix proposed in https://github.com/envoyproxy/envoy/pull/12701.