HNC: Implement validation webhook for HierarchicalNamespace (HNS)

[yiqigao217]

This is part of #457

The HNS webhook should deny the requests to:

• Create, if the parent namespace is not allowed to create subnamespaces
• Create, if the HNS name is conflict with currently existing namespaces
• Delete, if the owned namespace doesn't allow cascading deletion

The namespace (core type) webhook should deny the request to:

• Delete an owned namespace. It will say "please delete the hns instance in the owner instead".
• Delete an owner namespace if it doesn't allow cascading deletion and has owned ns.

The deletion/updating of an hc instance will not be implemented using webhook, but just RBAC rules. See #596

[jiachengxu]

Hi @yiqigao217 @adrianludwin I am interested in this issue, and I have experience with webhook, can I try to work on this?

[yiqigao217]

Hi @jiachengxu , sorry that I almost finished this one and will send out a PR shortly.