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_source_git.go

@@ -33,6 +33,7 @@ var exportSourceGitCmd = &cobra.Command{
 
   # Export a GitRepository source including the SSH key pair or basic auth credentials
   flux export source git my-private-repo --with-credentials > source.yaml`,
+	ValidArgsFunction: resourceNamesCompletionFunc(sourcev1.GroupVersion.WithKind(sourcev1.GitRepositoryKind)),
 	RunE: exportWithSecretCommand{
 		object: gitRepositoryAdapter{&sourcev1.GitRepository{}},
 		list:   gitRepositoryListAdapter{&sourcev1.GitRepositoryList{}},