[Platform] Add API for Kubernetes provider configuration discovery

[bhavin192]

• This API tries to fetch the details which can be used to pre-fill the data during Kubernetes provider creation (should work on OpenShift or Tanzu as well). The returned JSON is similar to what we use for the create call. Please refer this issue for more details: [Platform] [k8s] Simplified k8s provider config for deploying into the same cluster as YW #7394
• Use blank string for KUBECONFIG if kubeconfig file is not provided. This allows us to in-cluster credentials (ServiceAccount) when running in the same cluster as of the target cluster. Can be used to simplify the current flow later, we won't need a separate kubeconfig in that case.
• Finds out the region and AZ based on node annotations.
• Takes the pull secret name and the storage class name from app config.

Scenarios tested:

• Ran platform inside Kubernetes, curl the API, it returns the expected JSON.
• Tested all the failure scenarios like missing config, no permissions to get secret, nodes etc.

A sample output:

{
  "code": "kubernetes",
  "name": null,
  "active": true,
  "config": {
    "KUBECONFIG_IMAGE_PULL_SECRET_NAME": "yugabyte-k8s-pull-secret",
    "KUBECONFIG_PULL_SECRET_NAME": "yugabyte-k8s-pull-secret",
    "KUBECONFIG_PULL_SECRET_CONTENT": "{\"apiVersion\":\"v1\",\"data\":{\".dockerconfigjson\":\"removed==\"},\"kind\":\"Secret\",\"metadata\":{\"annotations\":{},\"name\":\"yugabyte-k8s-pull-secret\"},\"type\":\"kubernetes.io/dockerconfigjson\"}"
  },
  "region": null,
  "regionList": [
    {
      "code": "asia-south1",
      "name": null,
      "latitude": 0.0,
      "longitude": 0.0,
      "zoneList": [
        {
          "code": "asia-south1-a",
          "name": "asia-south1-a",
          "config": {
            "STORAGE_CLASS": "standard"
          }
        },
        {
          "code": "asia-south1-c",
          "name": "asia-south1-c",
          "config": {
            "STORAGE_CLASS": "standard"
          }
        }
      ],
      "config": null
    }
  ]
}

Open Questions

• Should the API endpoint as well as the methods be called something like /customers/:cUUID/providers/kubernetes_discovery?

This PR is backend part for the #7394

[iSignal]

Looks good! Do update the Test Plan for the PR description with a sample response and the test environment.

Most of the comments are about code organization.

[iSignal]

Looks good! Sorry for the delay, github PRs don't get put back in the "review" list unless you re-request the review.

[sb-yb]

We already use guava that has all the necessary collection required by this change.
Can you please use guava and not add yet another dependency?
@bhavin192

[bhavin192]

Sure, I wasn't aware of that. I will work on removing that and using guava. I will create a new PR shortly.