feat: Add http proxy support

[victornguen]

Motivation

Resolves #2298

This PR adds support for HTTP proxying to Tonic clients

Solution

This PR implements HTTP proxy support for Tonic clients by leveraging [hyper-util]'s built-in proxy functionality, specifically the Tunnel type for HTTP CONNECT proxying. The solution includes:

• Proxy configuration API:

  • Endpoint::proxy_uri() allows explicit proxy configuration.
  • Endpoint::proxy_from_env(true) enables automatic proxy detection via environment variables (http_proxy, https_proxy, no_proxy).

• Proxy resolution logic:

  • Explicit proxy configuration takes precedence over environment variables.
  • no_proxy is respected for bypassing the proxy for specific hosts.

• Tests:

  • Tests verify explicit proxy configuration, environment variable detection, no_proxy bypass, and precedence rules.
  • Tests use a mock proxy server to ensure actual proxy connections are made or bypassed as expected.
  • Test isolation is enforced to avoid environment variable leakage between tests.

This change is backward compatible.

[victornguen]

@dfawley @LucioFranco please take a look

[ganluo960214]

@victornguen i also want this feature. i using Endpoint.connect_with_connector. but got error can't resolve.

    async fn connect(endpoint: Endpoint) -> Result<TonicChannel, tonic::transport::Error> {
        let channel = endpoint.connect().await?;
        Ok(channel)
        // get the https_proxy env var
        if let Ok(proxy) = std::env::var("HTTPS_PROXY") {
            let proxy = {
                let proxy_uri = proxy.parse().unwrap();
                let proxy = Proxy::new(Intercept::All, proxy_uri);
                let connector = HttpConnector::new();
                let proxy_connector = ProxyConnector::from_proxy(connector, proxy).unwrap();
                proxy_connector
            };
            let channel = endpoint.connect_with_connector(proxy).await?;
            Ok(channel)
        } else {
            let channel = endpoint.connect().await?;
            Ok(channel)
        }
    }

TonicTransport(tonic::transport::Error(Transport, ConnectError(Custom { kind: InvalidData, error: InvalidMessage(InvalidContentType) })))