Filesystem based MainDomLock & extract interface for MainDomKey generation (#12037)

* Extract MainDomKey generation to its own class to ease customization.

Also add discriminator config value to GlobalSettings for advanced users.
Prevents a mandatory custom implementation, should be good enough for
the vast majority of use cases.

* Prevent duplicate runs of ScheduledPublishing during slot swap.

* Add filesystem based MainDomLock

src/Umbraco.Core/Configuration/Models/GlobalSettings.cs

@@ -137,6 +137,14 @@ namespace Umbraco.Cms.Core.Configuration.Models
         /// </summary>
         public string MainDomLock { get; set; } = string.Empty;
 
+        /// <summary>
+        /// Gets or sets a value to discriminate MainDom boundaries.
+        /// <para>
+        /// Generally the default should suffice but useful for advanced scenarios e.g. azure deployment slot based zero downtime deployments.
+        /// </para>
+        /// </summary>
+        public string MainDomKeyDiscriminator { get; set; } = string.Empty;
+
         /// <summary>
         /// Gets or sets the telemetry ID.
         /// </summary>