Add end-to-end testing

[xrstf]

Summary

This PR adds the first e2e tests for the init-agent, ensuring the basic functionality.

I also included a fix for the multicluster code: It was accidentally using the kcp kubeconfig, i.e. it was creating new objects inside new workspaces using the /clusters/... endpoint directly (which worked in my local testing becaue I used the admin kubeconfig from kcp). Instead the initcontroller has to use the virtual workspace endpoint (/services/initws/clusters/....). Since this URL is not available to the initcontroller (the multicluster provider has no way to pass it down to the controllers), all the controller has is mgr.GetCluster().GetClient().

So to solve this, there is no longer a ClusterApplier, and the applier itself is not containing a client at all anymore. Instead it gets it passed in the Apply() call. This is technically not the abstraction I head in mind for the applier, but alas.

What Type of PR Is This?

/kind feature

Release Notes

NONE

[kcp-ci-bot]

@xrstf: The label(s) kind/feature cannot be applied, because the repository doesn't have them.

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Summary

This PR adds the first e2e tests for the init-agent, ensuring the basic functionality.

What Type of PR Is This?

/kind feature

Release Notes

NONE

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[kcp-ci-bot]

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