[Feature] Support Client Status Discovery Service (CSDS)

*Title*: *[Feature] Support Client Status Discovery Service (CSDS)*

*Description*:

[Client Status Discovery Service (CSDS)](https://github.com/envoyproxy/envoy/blob/master/api/envoy/service/status/v4alpha/csds.proto) is a service that exposes xDS config of a given client. It’s commonly used to query control planes for the synced xDS config of a particular sidecar proxy. However, it can also be used to query Envoy for its received xDS configuration.

As a xDS client, Envoy can dump xDS config and expose internal status of each xDS resource (https://github.com/envoyproxy/envoy/pull/13121). The xDS config dump will share the same proto definition as admin interface. It will be an alternative way of getting config dump. As for internal status, it describes if the Envoy accepts or rejects or is pending on a specific xDS resource.

*Relevant Links*:
* For Googler, [go/grpc-csds](http://go/grpc-csds)
* [CSDS proto definition](https://github.com/envoyproxy/envoy/blob/master/api/envoy/service/status/v4alpha/csds.proto)

CC @htuch for more potential benefits.