feat: Support --header for CLI commands to support proxies (#4008)

Fixes #3527.

cli/root_test.go

@@ -2,9 +2,12 @@ package cli_test
 
 import (
 	"bytes"
+	"net/http"
+	"net/http/httptest"
 	"testing"
 
 	"github.com/spf13/cobra"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/xerrors"
 
@@ -129,4 +132,25 @@ func TestRoot(t *testing.T) {
 		require.Contains(t, output, buildinfo.Version(), "has version")
 		require.Contains(t, output, buildinfo.ExternalURL(), "has url")
 	})
+
+	t.Run("Header", func(t *testing.T) {
+		t.Parallel()
+
+		done := make(chan struct{})
+		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			assert.Equal(t, "wow", r.Header.Get("X-Testing"))
+			w.WriteHeader(http.StatusGone)
+			select {
+			case <-done:
+				close(done)
+			default:
+			}
+		}))
+		defer srv.Close()
+		buf := new(bytes.Buffer)
+		cmd, _ := clitest.New(t, "--header", "X-Testing=wow", "login", srv.URL)
+		cmd.SetOut(buf)
+		// This won't succeed, because we're using the login cmd to assert requests.
+		_ = cmd.Execute()
+	})
 }