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/delete_alertprovider.go

@@ -28,6 +28,7 @@ var deleteAlertProviderCmd = &cobra.Command{
 	Long:  "The delete alert-provider command removes the given Provider from the cluster.",
 	Example: `  # Delete a Provider and the Kubernetes resources created by it
   flux delete alert-provider slack`,
+	ValidArgsFunction: resourceNamesCompletionFunc(notificationv1.GroupVersion.WithKind(notificationv1.ProviderKind)),
 	RunE: deleteCommand{
 		apiType: alertProviderType,
 		object:  universalAdapter{&notificationv1.Provider{}},