feat(xds): initial crate structure

[YutaoMa]

Initialize the crate scaffolding for xds-client.

Refs: #2444

Motivation

In recent years, the official gRPC implementations have been adding out-of-box service discovery and load-balancing capabilities via the xDS protocol. This enables gRPC users to build a service mesh without having to pay the overhead of proxies.

My team have been using gRPC and xDS massively in our production environment. For Rust, we have recently added xDS support to our internal Tonic-based gRPC framework, and it achieved significant performance improvements over the Proxy-based service mesh implementation. We’d like to contribute our xDS implementation to Tonic as well as the ongoing Rust gRPC effort, so that other Rust users can also benefit from the lightweight proxyless service mesh.

Solution

This PR includes only the scaffolding of the new crate xds-client. While currently unpublished, the plan is to continue developing it along with tonic and the upcoming grpc-rust, so that eventually we'll have a working xDS implementation in both.

[gu0keno0]

LGTM!

[LucioFranco]

LGTM, we are going to need to think about how we setup CI (and maybe there is a bigger convo here realted to getting grpc-rust tested as well). Maybe this is a good item to tackle next while the code here is still small.