feat: Allow iox clients to specify connector

This allows to implement custom transports for iox clients (such as socks5 proxies, see #2683).

client_util/src/connection.rs

@@ -4,7 +4,8 @@ use http::{uri::InvalidUri, HeaderValue, Uri};
 use std::convert::TryInto;
 use std::time::Duration;
 use thiserror::Error;
-use tonic::transport::Endpoint;
+use tonic::transport::{Channel, Endpoint};
+use tower::make::MakeConnection;
 
 /// The connection type used for clients
 pub type Connection = SetRequestHeadersService<tonic::transport::Channel>;
@@ -89,22 +90,44 @@ impl std::default::Default for Builder {
 impl Builder {
     /// Construct the [`Connection`] instance using the specified base URL.
     pub async fn build<D>(self, dst: D) -> Result<Connection>
+    where
+        D: TryInto<Uri, Error = InvalidUri> + Send,
+    {
+        let endpoint = self.create_endpoint(dst)?;
+        let channel = endpoint.connect().await?;
+        Ok(self.compose_middleware(channel))
+    }
+
+    /// Construct the [`Connection`] instance using the specified base URL and custom connector.
+    pub async fn build_with_connector<D, C>(self, dst: D, connector: C) -> Result<Connection>
+    where
+        D: TryInto<Uri, Error = InvalidUri> + Send,
+        C: MakeConnection<Uri> + Send + 'static,
+        C::Connection: Unpin + Send + 'static,
+        C::Future: Send + 'static,
+        Box<dyn std::error::Error + Send + Sync>: From<C::Error> + Send + 'static,
+    {
+        let endpoint = self.create_endpoint(dst)?;
+        let channel = endpoint.connect_with_connector(connector).await?;
+        Ok(self.compose_middleware(channel))
+    }
+
+    fn create_endpoint<D>(&self, dst: D) -> Result<Endpoint>
     where
         D: TryInto<Uri, Error = InvalidUri> + Send,
     {
         let endpoint = Endpoint::from(dst.try_into()?)
-            .user_agent(self.user_agent)?
+            .user_agent(&self.user_agent)?
             .connect_timeout(self.connect_timeout)
             .timeout(self.timeout);
+        Ok(endpoint)
+    }
 
-        let channel = endpoint.connect().await?;
-
+    fn compose_middleware(self, channel: Channel) -> Connection {
         // Compose channel with new tower middleware stack
-        let channel = tower::ServiceBuilder::new()
+        tower::ServiceBuilder::new()
             .layer(SetRequestHeadersLayer::new(self.headers))
-            .service(channel);
-
-        Ok(channel)
+            .service(channel)
     }
 
     /// Set the `User-Agent` header sent by this client.