client: Try to re-introduce HTTP/HTTPS proxy (#31002)

When building for the `x86_64-unknown-linux-musl` target, the default
`openssl-dev` is compiled for the GNU toolchain, which causes a build
error due to missing OpenSSL. This PR fixes the issue by avoiding the
use of OpenSSL on non-macOS and non-Windows platforms.


Release Notes:

- N/A

crates/client/src/proxy.rs

@@ -0,0 +1,66 @@
+//! client proxy
+
+mod http_proxy;
+mod socks_proxy;
+
+use anyhow::{Context as _, Result};
+use http_client::Url;
+use http_proxy::{HttpProxyType, connect_http_proxy_stream, parse_http_proxy};
+use socks_proxy::{SocksVersion, connect_socks_proxy_stream, parse_socks_proxy};
+
+pub(crate) async fn connect_proxy_stream(
+    proxy: &Url,
+    rpc_host: (&str, u16),
+) -> Result<Box<dyn AsyncReadWrite>> {
+    let Some(((proxy_domain, proxy_port), proxy_type)) = parse_proxy_type(proxy) else {
+        // If parsing the proxy URL fails, we must avoid falling back to an insecure connection.
+        // SOCKS proxies are often used in contexts where security and privacy are critical,
+        // so any fallback could expose users to significant risks.
+        anyhow::bail!("Parsing proxy url failed");
+    };
+
+    // Connect to proxy and wrap protocol later
+    let stream = tokio::net::TcpStream::connect((proxy_domain.as_str(), proxy_port))
+        .await
+        .context("Failed to connect to proxy")?;
+
+    let proxy_stream = match proxy_type {
+        ProxyType::SocksProxy(proxy) => connect_socks_proxy_stream(stream, proxy, rpc_host).await?,
+        ProxyType::HttpProxy(proxy) => {
+            connect_http_proxy_stream(stream, proxy, rpc_host, &proxy_domain).await?
+        }
+    };
+
+    Ok(proxy_stream)
+}
+
+enum ProxyType<'t> {
+    SocksProxy(SocksVersion<'t>),
+    HttpProxy(HttpProxyType<'t>),
+}
+
+fn parse_proxy_type<'t>(proxy: &'t Url) -> Option<((String, u16), ProxyType<'t>)> {
+    let scheme = proxy.scheme();
+    let host = proxy.host()?.to_string();
+    let port = proxy.port_or_known_default()?;
+    let proxy_type = match scheme {
+        scheme if scheme.starts_with("socks") => {
+            Some(ProxyType::SocksProxy(parse_socks_proxy(scheme, proxy)))
+        }
+        scheme if scheme.starts_with("http") => {
+            Some(ProxyType::HttpProxy(parse_http_proxy(scheme, proxy)))
+        }
+        _ => None,
+    }?;
+
+    Some(((host, port), proxy_type))
+}
+
+pub(crate) trait AsyncReadWrite:
+    tokio::io::AsyncRead + tokio::io::AsyncWrite + Unpin + Send + 'static
+{
+}
+impl<T: tokio::io::AsyncRead + tokio::io::AsyncWrite + Unpin + Send + 'static> AsyncReadWrite
+    for T
+{
+}