param type of _ClientFactoryMixin.from_service_account_info method is wrong #162
Description
It seems that the info param type is wrong.
bigquery.Client.from_service_account_info method passes the info param to service_account.Credentials.from_service_account_info method.
service_account.Credentials.from_service_account_info method requires Mapping[str, str] type.
This method raises AttributeError if I pass a json string as info.
AttributeError: 'str' object has no attribute 'keys'