Use proper scheduler that honors the parallel task limit (#14281)

MediaBrowser.Controller/LibraryTaskScheduler/ILimitedConcurrencyLibraryScheduler.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using MediaBrowser.Model.Configuration;
+
+namespace MediaBrowser.Controller.LibraryTaskScheduler;
+
+/// <summary>
+/// Provides a shared scheduler to run library related tasks based on the <see cref="ServerConfiguration.LibraryScanFanoutConcurrency"/>.
+/// </summary>
+public interface ILimitedConcurrencyLibraryScheduler
+{
+    /// <summary>
+    /// Enqueues an action that will be invoked with the set data.
+    /// </summary>
+    /// <typeparam name="T">The data Type.</typeparam>
+    /// <param name="data">The data.</param>
+    /// <param name="worker">The callback to process the data.</param>
+    /// <param name="progress">A progress reporter.</param>
+    /// <param name="cancellationToken">Stop token.</param>
+    /// <returns>A task that finishes when all data has been processed by the worker.</returns>
+    Task Enqueue<T>(T[] data, Func<T, IProgress<double>, Task> worker, IProgress<double> progress, CancellationToken cancellationToken);
+}