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/Configuration/Models/HealthChecksNotificationMethodSettings.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+using Umbraco.Core.HealthCheck;
+
+namespace Umbraco.Core.Configuration.Models
+{
+    public class HealthChecksNotificationMethodSettings
+    {
+        public bool Enabled { get; set; } = false;
+
+        public HealthCheckNotificationVerbosity Verbosity { get; set; } = HealthCheckNotificationVerbosity.Summary;
+
+        public bool FailureOnly { get; set; } = false;
+
+        public IDictionary<string, string> Settings { get; set; } = new Dictionary<string, string>();
+    }
+}