Epic: Create and maintain popular vcluster plugins

[FabianKramm]

Use Case

Starting with v0.9.0 we have a greatly improvement plugin experience in vcluster and want to provide some plugins for very commonly used operators such as crossplane, argo cd or cert manager. With plugins it is possible to reuse operators of the host cluster inside each vcluster to avoid an extra installation there. Other use cases for plugins are also thinkable such as: a generic CRD syncer or an admission plugin.