Move database cache rebuild to a background task with polling (#18496)

* Use background queue for database cache rebuild and track rebuilding status.

* Updated OpenApi.json and client-side types.

* Updated client to poll for completion of database rebuild.

* Update src/Umbraco.Cms.Api.Management/Controllers/PublishedCache/RebuildPublishedCacheController.cs

Co-authored-by: Sven Geusens <sge@umbraco.dk>

* Removed unnecessary obsolete attribute.

---------

Co-authored-by: Sven Geusens <sge@umbraco.dk>

src/Umbraco.Core/PublishedCache/IDatabaseCacheRebuilder.cs

@@ -1,8 +1,33 @@
-namespace Umbraco.Cms.Core.PublishedCache;
+namespace Umbraco.Cms.Core.PublishedCache;
 
+/// <summary>
+/// Defines operations for rebuild of the published content cache in the database.
+/// </summary>
 public interface IDatabaseCacheRebuilder
 {
+    /// <summary>
+    /// Indicates if the database cache is in the process of being rebuilt.
+    /// </summary>
+    /// <returns></returns>
+    bool IsRebuilding() => false;
+
+    /// <summary>
+    /// Rebuilds the database cache.
+    /// </summary>
+    [Obsolete("Use the overload with the useBackgroundThread parameter. Scheduled for removal in Umbraco 17.")]
     void Rebuild();
 
+    /// <summary>
+    /// Rebuilds the database cache, optionally using a background thread.
+    /// </summary>
+    /// <param name="useBackgroundThread">Flag indicating whether to use a background thread for the operation and immediately return to the caller.</param>
+    void Rebuild(bool useBackgroundThread)
+#pragma warning disable CS0618 // Type or member is obsolete
+        => Rebuild();
+#pragma warning restore CS0618 // Type or member is obsolete
+
+    /// <summary>
+    /// Rebuids the database cache if the configured serializer has changed.
+    /// </summary>
     void RebuildDatabaseCacheIfSerializerChanged();
 }