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/get_image_policy.go

@@ -34,6 +34,7 @@ var getImagePolicyCmd = &cobra.Command{
 
  # List image policies from all namespaces
   flux get image policy --all-namespaces`,
+	ValidArgsFunction: resourceNamesCompletionFunc(imagev1.GroupVersion.WithKind(imagev1.ImagePolicyKind)),
 	RunE: func(cmd *cobra.Command, args []string) error {
 		get := getCommand{
 			apiType: imagePolicyType,