Update addon implementation

[Adirio]

Both the walkthrough found in this repo, and the implementation found in https://github.com/kubernetes-sigs/kubebuilder are out of date. Addon should be implemented as a kubebuilder plugin that builds over the base plugin functionality to replace the required files and create the new ones.

Wether this plugin should be implemented in this repo or in https://github.com/kubernetes-sigs/kubebuilder is still an open question. I can implement the addon plugin if some member of this repo specifies the steps requires to update a project built with a newer version of kubebuilder, either the last release (2.3.1) or building it from master. In any case, also provide the output of kubebuilder version.

[fejta-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Send feedback to sig-contributor-experience at kubernetes/community.
/lifecycle stale

[Adirio]

/lifecycle frozen

[Adirio]

Related to kubernetes-sigs/kubebuilder#1991