Kubebuilder phase 2 plugin

The introduction of external plugin extends the functionality of the Kubebuilder CLI tool by allowing users to create and use custom plugins. These plugins can be written in any programming language, as long as they implement the interface specifications to interact with Kubebuilder. This proposal aims to provide best practices based on tutorials, testing and documentation. It also suggests the kube-state-metrics plugin as a sample project, which would scaffold the configuration for KSM to load and populate metrics for CR/CRD. The overall efforts are attempted to guide Kubernetes developers to learn how to write, install, and use their own Kubebuilder plugins.