aws: Fix proxy dial to pick all proxies

The current proxy dial function does not consider proxy
information provided in some environment variables like
http_proxy etc when checking if the service principal
provided is reachable. Adding a function that checks a few
env variables and dials the service to see if it is reachable.

pkg/asset/installconfig/aws/validation.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"net"
+	"net/http"
 	"net/url"
 	"sort"
 	"strings"
@@ -329,12 +330,28 @@ func validateEndpointAccessibility(endpointURL string) error {
 	if port == "" {
 		port = "https"
 	}
-	conn, err := proxy.Dial(context.Background(), "tcp", net.JoinHostPort(URL.Hostname(), port))
-	if err != nil {
-		return err
+	err = proxyDialURL(context.Background(), net.JoinHostPort(URL.Hostname(), port))
+	return err
+}
+
+func proxyDialURL(ctx context.Context, URL string) error {
+	conn, err := proxy.Dial(context.Background(), "tcp", URL)
+	if err == nil {
+		conn.Close()
+		return nil
+	}
+	// DefaultClient picks the http and https proxy environment variables.
+	req, err := http.NewRequestWithContext(context.Background(), "CONNECT", URL, nil)
+	if err == nil {
+		resp, err := http.DefaultClient.Do(req)
+		if err == nil {
+			if resp.StatusCode == 200 {
+				return nil
+			}
+			return fmt.Errorf("error reaching service endpoint %s: %s", URL, resp.Status)
+		}
 	}
-	conn.Close()
-	return nil
+	return err
 }
 
 var requiredServices = []string{