feat(chart): make ClusterIssuer configurable and ingress-class aware

[MoisesGSalas]

Add a clusterIssuer values section so the
harmony-letsencrypt-global ClusterIssuer can be toggled and its ACME HTTP01 ingress class can be customized.

• Fix the Traefik provider key casing from kubernetesIngressNginx to kubernetesIngressNGINX so it validates against the Traefik sub-chart schema.

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[MoisesGSalas]

Hi @gabor-boros, this is a small PR with a few fixes. I want to make a release before completely removing ingress-nginx (#194 WIP).