Add version checking when executing extensions

Building on logic from the `gh ext list` for retrieving and assessing extension release information, this commit enhances the logic around invoking extensions to check for new releases.

Using the same user experience from checking `gh` version, this should only output information when the extension is used and gives the user information on how to upgrade depending on the type of extension and whether it is pinned or not.

```shell
andrewfeller@Andrews-MacBook-Pro cli % gh ext install dlvhdr/gh-dash --pin v4.6.0
✓ Installed extension dlvhdr/gh-dash
✓ Pinned extension at v4.6.0
andrewfeller@Andrews-MacBook-Pro cli % ./bin/gh dash

A new release of dash is available: 4.6.0 → 4.7.0
To upgrade, run: gh extension upgrade dash --force
https://github.com/dlvhdr/gh-dash
```

pkg/cmd/root/extension.go

@@ -4,20 +4,45 @@ import (
 	"errors"
 	"fmt"
 	"os/exec"
+	"strings"
 
 	"github.com/cli/cli/v2/pkg/extensions"
 	"github.com/cli/cli/v2/pkg/iostreams"
+	"github.com/mgutz/ansi"
 	"github.com/spf13/cobra"
 )
 
 type ExternalCommandExitError struct {
 	*exec.ExitError
 }
 
+type extensionReleaseInfo struct {
+	CurrentVersion string
+	LatestVersion  string
+	Pinned         bool
+	URL            string
+}
+
 func NewCmdExtension(io *iostreams.IOStreams, em extensions.ExtensionManager, ext extensions.Extension) *cobra.Command {
+	// Setup channel containing information about potential latest release info
+	updateMessageChan := make(chan *extensionReleaseInfo)
+
 	return &cobra.Command{
 		Use:   ext.Name(),
 		Short: fmt.Sprintf("Extension %s", ext.Name()),
+		// PreRun handles looking up whether extension has a latest version only when the command is ran.
+		PreRun: func(c *cobra.Command, args []string) {
+			go func() {
+				if ext.UpdateAvailable() {
+					updateMessageChan <- &extensionReleaseInfo{
+						CurrentVersion: ext.CurrentVersion(),
+						LatestVersion:  ext.LatestVersion(),
+						Pinned:         ext.IsPinned(),
+						URL:            ext.URL(),
+					}
+				}
+			}()
+		},
 		RunE: func(c *cobra.Command, args []string) error {
 			args = append([]string{ext.Name()}, args...)
 			if _, err := em.Dispatch(args, io.In, io.Out, io.ErrOut); err != nil {
@@ -29,6 +54,24 @@ func NewCmdExtension(io *iostreams.IOStreams, em extensions.ExtensionManager, ex
 			}
 			return nil
 		},
+		// PostRun handles communicating extension release information if found
+		PostRun: func(c *cobra.Command, args []string) {
+			releaseInfo := <-updateMessageChan
+			if releaseInfo != nil {
+				stderr := io.ErrOut
+				fmt.Fprintf(stderr, "\n\n%s %s → %s\n",
+					ansi.Color(fmt.Sprintf("A new release of %s is available:", ext.Name()), "yellow"),
+					ansi.Color(strings.TrimPrefix(releaseInfo.CurrentVersion, "v"), "cyan"),
+					ansi.Color(strings.TrimPrefix(releaseInfo.LatestVersion, "v"), "cyan"))
+				if releaseInfo.Pinned {
+					fmt.Fprintf(stderr, "To upgrade, run: gh extension upgrade %s --force\n", ext.Name())
+				} else {
+					fmt.Fprintf(stderr, "To upgrade, run: gh extension upgrade %s\n", ext.Name())
+				}
+				fmt.Fprintf(stderr, "%s\n\n",
+					ansi.Color(releaseInfo.URL, "yellow"))
+			}
+		},
 		GroupID: "extension",
 		Annotations: map[string]string{
 			"skipAuthCheck": "true",