[configgrpc] enable xds support

Is your feature request related to a problem? Please describe.

Our gRPC load-balancing infrastructure relies on in-process gRPC xDS, rather than a sidecar or more traditional load-balancer. We're seeing stale endpoints being dialled as things scale up/down due to the configuration window on the round-robin service not being quick enough to pickup the changes.

Describe the solution you'd like

configgrpc to import _ "google.golang.org/grpc/xds" enabling endpoint to be set with xds:///{endpoint} and the balancer config to be left empty.