Support xDS server subscription to client resources

In order to be able to gain knowledge of resource readiness (https://github.com/envoyproxy/envoy/issues/5174), a management server today needs to resort to OOB techniques to learn whether a listener (for example) has successfully been bound to a port. There are other examples of this, for example resources failing to warm due to dependency failure. It's not sufficient to rely on xDS's inbuilt ACK/NACK mechanism, since these events often occur long after `DiscoveryResponse` acknowledgement.

We've been brainstorming approaches here and one that seemed highly attractive in UDPA-TP discussions was to make xDS fully symmetric (similar to what MCP does), allowing the management server to subscribe to resources produced from the client. This reverse pub-sub would then allow for details that are today made available in configdump or CSDS to be supplied directly to a management server inline in an ADS stream.

Ideally we have a design doc to start this work that examines how we can retrofit this into v3 xDS so that we can gain benefit today.