Serve Media and App_Plugins using WebRootFileProvider (and allow changing the physical media path) (#11783)

* Allow changing UmbracoMediaPath to an absolute path. Also ensure Imagesharp are handing requests outside of the wwwroot folder.

* Let UmbracoMediaUrl fallback to UmbracoMediaPath when empty

* Add FileSystemFileProvider to expose an IFileSystem as IFileProvider

* Replace IUmbracoMediaFileProvider with IFileProviderFactory implementation

* Fix issue resolving relative paths when media URL has changed

* Remove FileSystemFileProvider and require explicitly implementing IFileProviderFactory

* Update tests (UnauthorizedAccessException isn't thrown anymore for rooted files)

* Update test to use UmbracoMediaUrl

* Add UmbracoMediaPhysicalRootPath global setting

* Remove MediaFileManagerImageProvider and use composited file providers

* Move CreateFileProvider to IFileSystem extension method

* Add rooted path tests

Co-authored-by: Ronald Barendse <ronald@barend.se>

src/Umbraco.Web.Common/Extensions/FileProviderExtensions.cs

@@ -0,0 +1,19 @@
+using System.Linq;
+using Microsoft.Extensions.FileProviders;
+
+namespace Umbraco.Extensions
+{
+    internal static class FileProviderExtensions
+    {
+        public static IFileProvider ConcatComposite(this IFileProvider fileProvider, params IFileProvider[] fileProviders)
+        {
+            var existingFileProviders = fileProvider switch
+            {
+                CompositeFileProvider compositeFileProvider => compositeFileProvider.FileProviders,
+                _ => new[] { fileProvider }
+            };
+
+            return new CompositeFileProvider(existingFileProviders.Concat(fileProviders));
+        }
+    }
+}