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.Core/DateTimeExtensions.cs

@@ -46,7 +46,7 @@ namespace Umbraco.Core
 
         /// <summary>
         /// Calculates the number of minutes from a date time, on a rolling daily basis (so if
-        /// date time is before the time, calculate onto next day)
+        /// date time is before the time, calculate onto next day).
         /// </summary>
         /// <param name="fromDateTime">Date to start from</param>
         /// <param name="scheduledTime">Time to compare against (in Hmm form, e.g. 330, 2200)</param>