refactor(health): move ICentralHealthAggregator and SiteHealthState to Commons (ScadaBridge#71)

Interface goes to Commons.Interfaces.Services and the state record to
Commons.Messages.Health, so CentralCommunicationActor no longer needs the
HealthMonitoring namespace. Implementation stays in HealthMonitoring.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/ZB.MOM.WW.ScadaBridge.CentralUI/ServiceCollectionExtensions.cs

@@ -1,3 +1,4 @@
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services;
 using Microsoft.AspNetCore.Components.Authorization;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Options;