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/SmtpSettings.cs

@@ -1,10 +1,22 @@
-using System;
-using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations;
 using System.Net.Mail;
 using Umbraco.Core.Configuration.Models.Validation;
 
 namespace Umbraco.Core.Configuration.Models
 {
+    /// <summary>
+    /// Matches MailKit.Security.SecureSocketOptions and defined locally to avoid having to take
+    /// thi
+    /// </summary>
+    public enum SecureSocketOptions
+    {
+        None = 0,
+        Auto = 1,
+        SslOnConnect = 2,
+        StartTls = 3,
+        StartTlsWhenAvailable = 4
+    }
+
     public class SmtpSettings : ValidatableEntryBase
     {
         [Required]
@@ -15,6 +27,8 @@ namespace Umbraco.Core.Configuration.Models
 
         public int Port { get; set; }
 
+        public SecureSocketOptions SecureSocketOptions { get; set; } = SecureSocketOptions.Auto;
+
         public string PickupDirectoryLocation { get; set; }
 
         public SmtpDeliveryMethod DeliveryMethod { get; set; } = SmtpDeliveryMethod.Network;