webhook config directory scaffolded via webhook plugin instead of init one

[camilamacedo86]

What do you want to happen?

The webhook directory be scaffold only/if/when I use the webhook plugin via the command kb create webhook

Is there currently another issue associated with this?

Yes, see: #1727. It has the same motivations.

Eng. Notes

• See that the scaffolds are done in : https://github.com/kubernetes-sigs/kubebuilder/blob/master/pkg/plugin/v3/scaffolds/init.go#L126-L128
• See that it could ONLY be addressed to v3+ plugin
• See that after this change when we run make generate to update the testdata then, the addon project for v3 would no longer have the webhook directories.

/kind feature

[camilamacedo86]

c/c @estroz @prafull01