Support passing `dict` attributes in `update_dict` of `update_asset`

[Flix6x]

The update_dict["attributes"] should be JSON serialized to avoid a server error, but we can be flexible here and do a json.dumps in case a dict is passed.

The server complains about a JSON object being a dict, but responds with a 500 status code, which could be improved, too.

• Create FlexMeasures issue.