Allow plugins to make use of the cobra PersistentPreRun hooks.

Previously a plugin which used these hooks would overwrite the top-level plugin
command's use of this hook, resulting in the dockerCli object not being fully
initialised.

Provide a function which plugins can use to chain to the required behaviour.
This required some fairly ugly arrangements to preserve state (which was
previously in-scope in `newPluginCOmmand`) to be used by the new function.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli-plugins/examples/helloworld/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"fmt"
 
 	"github.com/docker/cli/cli-plugins/manager"
@@ -18,16 +19,33 @@ func main() {
 				fmt.Fprintln(dockerCli.Out(), "Goodbye World!")
 			},
 		}
+		apiversion := &cobra.Command{
+			Use:   "apiversion",
+			Short: "Print the API version of the server",
+			RunE: func(_ *cobra.Command, _ []string) error {
+				cli := dockerCli.Client()
+				ping, err := cli.Ping(context.Background())
+				if err != nil {
+					return err
+				}
+				fmt.Println(ping.APIVersion)
+				return nil
+			},
+		}
 
 		cmd := &cobra.Command{
 			Use:   "helloworld",
 			Short: "A basic Hello World plugin for tests",
+			// This is redundant but included to exercise
+			// the path where a plugin overrides this
+			// hook.
+			PersistentPreRunE: plugin.PersistentPreRunE,
 			Run: func(cmd *cobra.Command, args []string) {
 				fmt.Fprintln(dockerCli.Out(), "Hello World!")
 			},
 		}
 
-		cmd.AddCommand(goodbye)
+		cmd.AddCommand(goodbye, apiversion)
 		return cmd
 	},
 		manager.Metadata{