Add dynamic completion suppport

This commit adds dynamic completion support for the following commands
and flags:

- `flux delete ...` command
- `flux export ...` command
- `flux get ...` command
- `flux reconcile ...` command
- `flux resume ...` command
- `flux suspend ...` command
- `--namespace` flag
- `--context` flag

Signed-off-by: Kazuki Suda <kazuki.suda@gmail.com>

cmd/flux/export_alertprovider.go

@@ -32,6 +32,7 @@ var exportAlertProviderCmd = &cobra.Command{
 
   # Export a Provider
   flux export alert-provider slack > slack.yaml`,
+	ValidArgsFunction: resourceNamesCompletionFunc(notificationv1.GroupVersion.WithKind(notificationv1.ProviderKind)),
 	RunE: exportCommand{
 		object: alertProviderAdapter{&notificationv1.Provider{}},
 		list:   alertProviderListAdapter{&notificationv1.ProviderList{}},