feat(grpc): Add resolver wrapper to handle HTTP CONNECT proxy configuration

[arjan-bal]

This PR includes changes from grpc/grpc-rust#2677.

It implements a resolver wrapper that determines proxy routing by checking the HTTPS_PROXY and NO_PROXY environment variables. By default, hyper-util matches cURL's behavior by falling back to ALL_PROXY when HTTPS_PROXY is unset. However, because Go's FromEnvironment and gRPC C++ ignore ALL_PROXY, gRPC Rust manually configures the Matcher to bypass it, ensuring cross-language consistency.

Resolution Flow
When HttpsProxyResolver::Builder builds a resolver for a target URI, it uses hyper_util::client::proxy::Matcher to evaluate the environment variables:

1. Direct Connection: If no proxy is required, it delegates resolution entirely to the wrapped child builder, bypassing the proxy resolver.
2. Proxied Connection: If a proxy is required, it creates an HttpsProxyResolver, which uses the child DNS resolver to resolve the proxy server's hostname instead of the target.
3. Attribute Injection: The HttpsProxyResolver intercepts resolution updates from the child resolver. It attaches the necessary proxy configuration (the original target authority and basic auth credentials) as attributes to each resolved proxy address.

In follow-up PRs, the subchannel will read these address attributes to wrap the channel credentials and carry out the HTTP CONNECT handshake prior to the standard credential handshake.

Internal design doc: go/grpc-rust-http-connect

[arjan-bal]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors the gRPC client to support arbitrary byte-based addresses (via ByteStr) instead of requiring valid UTF-8 strings, enabling support for non-UTF-8 unix socket paths. It also introduces an HTTP CONNECT proxy resolver that intercepts DNS resolution and applies proxy configurations based on environment variables. The review feedback suggests bypassing proxy resolution entirely for non-DNS schemes (like unix or inmemory), safely handling empty NO_PROXY environment variables, and updating tests to verify that unix paths correctly bypass the proxy.