API to send/receive app-specific XDS configs/messages

APIs to interact with the XDS layer would be extremely useful for mobile - in particular a method to allow the native app to make an XDS request with an app-specified type, and to get back the opaque response from the server and ack/nack.

For mobile it is better to reduce the number of active TCP connections ( and associated keep-alives ), such API would allow the XDS server to also push app-specific configs, or configs for filters that are implemented using the app extension APIs. If this is combined with adding an opaque payload to the XDS request, the XDS connection can be used to multiplex a light push messaging mechanism, so apps can keep only one socket open.