feature: KongClusterPlugins for sharing plugin configuration across namespaces

[hbagdi]

Currently KongPlugins can only be accessed from the namespace they reside in, as they have Namespaced scope. In many scenarios, it may make sense to apply the same KongPlugin to Ingresses in different namespaces, e.g. for separate teams that manage their Ingress/Service definitions in a team-specific namespace, but should use an organization-wide OIDC config for authenticating clients.

A Cluster scope KongPlugin CRD would allow easy plugin configuration sharing across an organization, without manually duplicating and synchronizing the definitions on a per-workspace basis.

cc @rainest