Support for gRPC with XDS

[stevenctl]

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

Testing with XDS resolver enabled would be very useful, especially when trying to compare:

• Plain gRPC
• gRPC with some proxy in front of it (e.g. Envoy, Linkerd proxy, etc)
• gRPC with XDS resolver enabled/TLS configs etc.

Describe the solution you'd like

Simply performing the side-effect import will allow xds:/// scheme to be used (assuming the user has the proper env variables for GRPC_XDS_BOOTSTRAP) set.

Additional context

gRPC has added support for configuration via XDS: https://github.com/grpc/proposal/blob/master/A27-xds-global-load-balancing.md

To use it as a client in Go, we simply need to add the following side-effect import:

// To install the xds resolvers and balancers. 
_ "google.golang.org/grpc/xds"

This just registers the builder, but doesn't do anything unless a bootstrap file is provided.

[bojand]

Hello I think this is addressed via #300 and should be available in 0.100.0. Feel free to reopen the issue if there are problems.