ResourceGroups() not supported with "Multi-value" mode

[adeverteuil]

Example error from query inspector:

{
  "xhrStatus": "complete",
  "request": {
    "method": "GET",
    "url": "api/datasources/proxy/2/azuremonitor/subscriptions/<guid>/resourceGroups/{rgA,rgB}/resources?api-version=2018-01-01"
  },
  "response": {
    "error": {
      "code": "InvalidResourceGroup",
      "message": "The provided resource group name '{rgA,rgB}' has these invalid characters: '{,}'. The name can only be a letter, digit, '-', '.', '(', ')' or '_'."
    }
  }
}

The API expects exactly one resource group name.

Possible improvements: should document it better and have a better error message.

[vasekch]

Duplicate with #60