Proxyless grpc via xDS

#### Feature Description

Support (or document) proxyless grpc via xDS. Relevant gRFCs:

https://github.com/grpc/proposal/blob/master/A27-xds-global-load-balancing.md
https://github.com/grpc/proposal/blob/master/A28-xds-traffic-splitting-and-routing.md
https://github.com/grpc/proposal/blob/master/A30-xds-v3.md

#### Use Case(s)

I would like to set up gRPC services to connect to consul's grpc endpoint with a bootstrap file: https://github.com/grpc/proposal/blob/master/A27-xds-global-load-balancing.md#xdsclient-and-bootstrap-file

I can then connect to any grpc service in my catalog via xds:///my-service-name

This should allow me to load balance across services without having to have envoy proxies, reducing operational complexity.

I tried this without success. My grpc service connects to consul's xDS service makes a discovery request but then times out. I know the gRPC endpoint is meant for consul connect and envoy but it doesn't seem much of a stretch to broaden the usage to proxyless grpc.

Start with the referenced A27, A28, and A30 gRFCs and the Consul xDS endpoint/bootstrap-file behavior. Reproduce the xds:///my-service-name discovery timeout, then define whether the outcome is proxyless gRPC support or documentation of the supported configuration and limitations.

Written by the indexing model from the issue text.