feature: add kcp-controller-manager standalone binary

[embik]

Feature Description

controllers in kcp can have a leader election since #2996. This means that only a single instance of controllers can (and should) be running.

For scaling and HA purposes, it is interesting to run multiple kcp instances that all serve the kcp workspaces in this shard. Same as in Kubernetes, it would be nice to be able to only run the "apiserver" portion of kcp with multiple replicas.

Proposed Solution

Implement a standalone kcp-controller-manager binary that implements the controller starting portion of kcp and implement a way to disable controllers in the kcp "core" binary.

Alternative Solutions

No response

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Additional Context

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