feat(xds): add public APIs for xds-client

[YutaoMa]

Motivation

As explained previously #2459 , we are upstreaming an implementation of xDS client in Rust, in order to support xDS based routing and load balancing in tonic / gRPC Rust eventually. This PR is a continuation of the effort, by exposing all the expected public APIs (traits, types, function signatures) first to lay out the design.

Solution

Here is an overview of all the public APIs we are adding in this PR:

File	Public Types
error.rs	Error, Result
resource.rs	Resource trait
transport.rs	Transport, TransportStream, DiscoveryRequest, DiscoveryResponse
runtime.rs	Runtime trait
config.rs	ClientConfig
watcher.rs	ResourceWatcher<T>, ResourceEvent<T>
client.rs	XdsClient, XdsClientBuilder

In case the reviewer is curious if the provided APIs could support the expected functionality of an xDS client, I have a working proof of concept that implements these APIs in my fork: https://github.com/YutaoMa/tonic/tree/yutaoma/xds-client-poc, it showcases the APIs working together with an implementation based on the tonic stack (tonic, tokio, prost).

[LucioFranco]

@YutaoMa CI is failing because we have a job that checks that we don't expose certain APIs publically so this should be addressed https://github.com/hyperium/tonic/actions/runs/20112284748/job/57712601774?pr=2464#step:7:69 ideally, we should not expose things publically that we don't own to avoid potential breaking changes in the future.

[YutaoMa]

@LucioFranco as suggested I've removed the dependency on the pre-1.0 futures crate and added bytes to metadata allowlist similar to other tonic crates.

[gu0keno0]

Some general questions, LGTM overall

[LucioFranco]

LGTM! Feel free to address things in follow ups, nice to see the start here!

[dfawley]

Sorry for the late review. In general this looks great, thanks! I have a few changes to suggest, and feel free to punt on some of them until later.