Netcore: Health check notifier hosted service (#9295)

* Implemented health check notifier as a hosted service.
Added validation to health check settings.

* Registered health check notifier as a hosted service.
Modified health check nested settings to use concrete classes to align with other configuration models.

* Resolved issues with email sending using development server.

* PR review comments and fixed failing unit test.

* Changed period and delay millisecond and hourly values to TimeSpans.
Changed configuration of first run time for health check notifications to use H:mm format.

* Set up SecureSocketOptions as a locally defined enum.

* Tightened up time format validation to verify input is an actual time (with hours and minutes only) and not a timespan.

* Aligned naming and namespace of health check configuration related classes with other configuration classes.

* Created constants for hex colors used in formatting health check results as HTML.

* Revert "Tightened up time format validation to verify input is an actual time (with hours and minutes only) and not a timespan."

This reverts commit f9bb8a7a825bcb58146879f18b47922e09453e2d.

* Renamed method to be clear validation is of a TimeSpan and not a time.

Co-authored-by: Bjarke Berg <mail@bergmania.dk>

src/Umbraco.Web.BackOffice/Extensions/UmbracoBuilderExtensions.cs

@@ -16,7 +16,8 @@ namespace Umbraco.Extensions
                 .WithMiniProfiler()
                 .WithMvcAndRazor()
                 .WithWebServer()
-                .WithPreview();
+                .WithPreview()
+                .WithHostedServices();
         }
 
         public static IUmbracoBuilder WithBackOffice(this IUmbracoBuilder builder)