[Config] Support listing sections

[jiasli]

Support listing sections.

This PR supports Azure/azure-cli#14436, where az config get can list all sections.

> az config get
{
  "cloud": [
    {
      "name": "name",
      "source": "C:\\Users\\jiasli\\.azure\\config",
      "value": "AzureCloud"
    }
  ],
  "defaults": [
    {
      "name": "group",
      "source": "C:\\Users\\jiasli\\.azure\\config",
      "value": "rg1"
    }
  ]
}

[arrownj]

🚢