support "--export-manifest" args

[ruyingzhe]

What would you like to be added:

support command:
"kubectl clusternet apply -f nginx.yaml --dry-run --export-manifest",
which will output the manifest yaml.

Why is this needed:

We try to use ArgoCD with Clusternet kubectl plugin, to execute the yaml from Gitrepo to clusters.
The ArgoCD plugin need to read the output of “kubectl clusternet “ ， and to sync the manifest yaml to cluster.