validator-api: add apy data to mixnodes endpoint (#1393)

* validator-api: compute and return APY for all mixnode bonds

* validator-api: tidy

* validator-api: handle the absence of storage

* validator-api: some comments

* validator-api: refinements to apy calc

* validator-api: extract out some calculations

* changelog: add note

validator-api/src/node_status_api/routes.rs

@@ -281,6 +281,7 @@ pub(crate) async fn get_mixnode_avg_uptime(
     }))
 }
 
+// DEPRECATED: the uptime is available as part of the `/mixnodes/detailed` endpoint
 #[openapi(tag = "status")]
 #[get("/mixnodes/avg_uptime")]
 pub(crate) async fn get_mixnode_avg_uptimes(