Manage webhook cert internally

[alculquicondor]

What would you like to be added:

#220 introduced a dependency on cert-manager.io to handle the certificates, as recommended by kubebuilder docs. It's useful to have our own certificate issuer internally.

Here is how HNC sets them up https://github.com/kubernetes-sigs/hierarchical-namespaces/blob/e3937b351d8cdc629f1063228236dd80413affe8/cmd/manager/main.go#L105

Why is this needed:

This simplifies deployment of kueue. cert-manager.io is a big dependency that we should try to avoid.

[kerthcet]

I'm eager for this feature for some poc purposes, so I'd like to take a look first.
/assign