Expose health status over RPC (#6274)

* Node-health to a separate crate.

* Initialize node_health outside of dapps.

* Expose health over RPC.

* Bring back 412 and fix JS.

* Add health to workspace and tests.

* Fix compilation without default features.

* Fix borked merge.

* Revert to generics to avoid virtual calls.

* Fix node-health tests.

* Add missing trailing comma.

js/src/api/rpc/parity/parity.js

@@ -385,6 +385,11 @@ export default class Parity {
       .then(outNumber);
   }
 
+  nodeHealth () {
+    return this._transport
+      .execute('parity_nodeHealth');
+  }
+
   nodeName () {
     return this._transport
       .execute('parity_nodeName');