Summary: Previously, we used a predefined list of regions in Kubernetes, which caused failures for custom regions. With this diff, users can now create and use custom regions as well.

Test Plan:
iTest pipeline
Tested by creating with name testRegion.
```
{
  "code": "kubernetes",
  "name": "example3",
  "details": {
    "cloudInfo": {
      "kubernetes": {
        "kubernetesImageRegistry": "quay.io/yugabyte/yugabyte",
        "kubernetesProvider": "custom",
        "kubernetesPullSecretContent": "apiVersion: v1\nkind: Secret\nmetadata:\n  name: yugabyte-k8s-emea\ndata:\n  .dockerconfigjson: redacted\ntype: kubernetes.io/dockerconfigjson",
        "kubernetesPullSecretName": "emea-secret.yaml",
        "kubernetesImagePullSecretName": "yugabyte-k8s-emea"
      }
    }
  },
  "regions": [
    {
      "code": "test-region",
      "name": "Test Region",
      "zones": [
        {
          "code": "testregion-1",
          "name": "testregion-1",
          "details": {
            "cloudInfo": {
              "kubernetes": {
                "kubeConfigContent": "apiVersion: v1\nclusters:\n- cluster:\n    certificate-authority-data: redacted\n    server: https://multicloud-aks-1-dns-chzr3ait.hcp.eastus.azmk8s.io:443\n  name: multicloud-aks-east-us\ncontexts:\n- context:\n    cluster: multicloud-aks-east-us\n    user: yugabyte-platform-universe-management-multicloud-aks-east-us\n  name: yugabyte-platform-universe-management-multicloud-aks-east-us\ncurrent-context: yugabyte-platform-universe-management-multicloud-aks-east-us\nkind: Config\npreferences: {}\nusers:\n- name: yugabyte-platform-universe-management-multicloud-aks-east-us\n  user:\n    token: eyJhbGciOiJSUzI1NiIsImtpZCI6Inc0VnRkWU5lUFhRLU52UkZzanBnazFoU2lRQUhDS2M4dWhLT21QMW5KMk0ifQ.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.0hPt8VLcgB5gQaYjMA_FoTU51_JqRMhrHYumCmlDSGw9Elmmn1spNhcVGn_Q0KnSmKBgcYPECU5HO61R0UzB4y5qFPuTFRo743IaHu3qFSWEKChPu314p8L6ZNqZYlACfnjgINdKpVr3fsaY_Y4mJ80PhbDdfn0qqfrV8G197XhAQgO0ebJ8h0UULvJl85eRsvN4cukmh5qnqw2b_Qtll7SrI1CMsqfi6M6op7HILoiWphm2PHZUOz2EW4eiPr7JOSTz1i5gOgFzIYwpT1mkO5Ppsbx4Jfu32pa4prNRzcOV8WOJKJUlOht3GDfCTQPWj5sOS7ortYwf-QUpsGY9uQgwYq8MFtC5-MZClKUaerEKYlHJ7dQNLM-eA3zQxzaEscO4FNgfht5Wp5_bBownbSAO2jFfppRkJQqrxjWNxSEPAQ7po8U1ph2L63GFYY8I7-FcZqpM54Y0YPajrjY7x4s-V35-ym8xDMUGvAuEyG5qQeD4K3JvHJg6cDWk993RFm7QdCzDa-nvCa5jYpxEadpPOUCicsd-vnSDYuJ6cAriIueUab2dUJHpYJMmJAeQRcfCSKwIMFdVJFQXXLAzfshrSYJAPeVzTOEkty9uMoQ5s-QHXs584c3J3CQUpIONtz7q7zYsr10eyNYnS48LSicTzu6bQDHlhMWWszQiXK0\n",
                "kubeConfigName": "kubeconf-eastus-1.conf",
                "kubePodAddressTemplate": "{pod_name}.{namespace}.svc.{cluster_domain}",
                "kubernetesStorageClass": "yb-sc",
                "overrides": "istioCompatibility:\n  enabled: true\nmulticluster:\n  createServicePerPod: true\nnodeSelector:\n  yugabyte: ybdb"
              }
            }
          }
        }
      ]
    }
  ]
}
```

Reviewers: anijhawan, anabaria, vkumar

Reviewed By: anijhawan

Differential Revision: https://phorge.dev.yugabyte.com/D45816