rest.Config: support configuring an explict proxy URL

With support of http, https, and socks5 proxy support. We already
support configuring this via environmnet variables, but this approach
becomes inconvenient dealing with multiple clusters on different
networks, that require different proxies to connect to. Most solutions
require wrapping clients (like kubectl) in bash scripts.

Part of: https://github.com/kubernetes/client-go/issues/351

Kubernetes-commit: f3f666d5f1f6f74a8c948a5c64af993696178244

tools/clientcmd/client_config_test.go

@@ -23,10 +23,10 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/imdario/mergo"
-
 	restclient "k8s.io/client-go/rest"
 	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
+
+	"github.com/imdario/mergo"
 )
 
 func TestMergoSemantics(t *testing.T) {
@@ -330,6 +330,84 @@ func TestCertificateData(t *testing.T) {
 	matchByteArg(keyData, clientConfig.TLSClientConfig.KeyData, t)
 }
 
+func TestProxyURL(t *testing.T) {
+	tests := []struct {
+		desc      string
+		proxyURL  string
+		expectErr bool
+	}{
+		{
+			desc: "no proxy-url",
+		},
+		{
+			desc:     "socks5 proxy-url",
+			proxyURL: "socks5://example.com",
+		},
+		{
+			desc:     "https proxy-url",
+			proxyURL: "https://example.com",
+		},
+		{
+			desc:     "http proxy-url",
+			proxyURL: "http://example.com",
+		},
+		{
+			desc:      "bad scheme proxy-url",
+			proxyURL:  "socks6://example.com",
+			expectErr: true,
+		},
+		{
+			desc:      "no scheme proxy-url",
+			proxyURL:  "example.com",
+			expectErr: true,
+		},
+		{
+			desc:      "not a url proxy-url",
+			proxyURL:  "chewbacca@example.com",
+			expectErr: true,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.proxyURL, func(t *testing.T) {
+
+			config := clientcmdapi.NewConfig()
+			config.Clusters["clean"] = &clientcmdapi.Cluster{
+				Server:   "https://localhost:8443",
+				ProxyURL: test.proxyURL,
+			}
+			config.AuthInfos["clean"] = &clientcmdapi.AuthInfo{}
+			config.Contexts["clean"] = &clientcmdapi.Context{
+				Cluster:  "clean",
+				AuthInfo: "clean",
+			}
+			config.CurrentContext = "clean"
+
+			clientBuilder := NewNonInteractiveClientConfig(*config, "clean", &ConfigOverrides{}, nil)
+
+			clientConfig, err := clientBuilder.ClientConfig()
+			if test.expectErr {
+				if err == nil {
+					t.Fatal("Expected error constructing config")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("Unexpected error constructing config: %v", err)
+			}
+
+			if test.proxyURL == "" {
+				return
+			}
+			gotURL, err := clientConfig.Proxy(nil)
+			if err != nil {
+				t.Fatalf("Unexpected error from proxier: %v", err)
+			}
+			matchStringArg(test.proxyURL, gotURL.String(), t)
+		})
+	}
+}
+
 func TestBasicAuthData(t *testing.T) {
 	username := "myuser"
 	password := "mypass" // Fake value for testing.