In the c# SDK, Attributes is an object without any properties or fields. It should be a dictionary. I believe this can be accomplished by adding "additionalProperties": true to the Attributes definition in the API specs file. I'm not sure if the values should be a free-form object, or if they can/should be limited to certain types.
https://swagger.io/docs/specification/data-models/dictionaries/
https://developer.salesforce.com/docs/marketing/marketing-cloud/guide/sendSMSMessageSingleRecipient.html (recipient.attributes)