Handle caBundleSecretRef in the shard controller

[BergCyrill]

Summary

This PR adds the handling of the provided secret in the shards spec at .spec.caBundleSecretRef to the shards external logical cluster kubeconfig. This behavior is described in the CRD but is currently not implemented. This leads to issues if the externalHostname does not have the same certificate issuer as the serverCA (e.g. front-proxy with lets encrypt certificate and the rootShard & shard with self signed certificates).
The controller tests are extended to test for the correct path in the certificateAuthority property of the kubeconfig.

What Type of PR Is This?

/kind bug

Related Issue(s)

/

Release Notes

Fix usage of caBundleSecretRef in external logical cluster kubeconfig for shards resources similar to the rootShard behavior

[kcp-ci-bot]

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Hi @BergCyrill. Thanks for your PR.

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