Caryatid: Improve RESTResponse::with_json to take T: Serialize

[sandtreader]

Suggestion from @SupernaviX to improve REST handlers - if RESTResponse::with_json() could take a T: Serialize, and returned a Result<()>, it could do the serialisation and error handling itself:

                            Some(stake) => match serde_json::to_string(&stake) {
                                Ok(body) => Ok(RESTResponse::with_json(200, &body)),
                                Err(error) => Err(anyhow!("{:?}", error)),
                            },

becomes

                            Some(stake) => RESTResponse::with_json(200, &stake)