epic: support writing multi-cluster controllers in upstream controller-runtime

[mirzakopic]

Description

Description

Objectives

• Developers should be able to use upstream controller-runtime (https://github.com/kubernetes-sigs/controller-runtime) plus a small kcp plugin to write kcp-aware controllers

Stories

• Pick up 📖 Add designs/multi-cluster.md kubernetes-sigs/controller-runtime#2746 and ✨ WIP: Cluster Provider and cluster-aware controllers kubernetes-sigs/controller-runtime#2726
• Develop a typed implementation MVP
• Work with Stefan Schimanski on finalising the code and proposal
• Get the implementation merged
• Develop a cluster provider plugin hosted in github.com/kcp-dev

Demo Required

Yes

Demo Steps

1. Showcase a kcp-aware controller using upstream controller-runtime

Migrated from openmcp/kcp-upstream#9