Add the ability to provide imagePullSecrets for private Registries or Registries with auth

[BergCyrill]

When trying to use the provided helm charts in an air-gapped environment (or) with a private registry using authentication it will fail. This PR adds the ability to optionally provide imagePullSecrets for every chart.

Tested it on a local kind cluster and validated the Deployment manifests with and without providing imagePullSecrets.

[kcp-ci-bot]

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[kcp-ci-bot]

Hi @BergCyrill. Thanks for your PR.

I'm waiting for a kcp-dev member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[embik]

/ok-to-test