v14: Rewrite static assets for the backoffice (#14472)

* add extension method to rewrite requests meant for backoffice static assets

* add backoffice rewrites to application builder

* embed the umbraco hash into the static assets requested on the backoffice

* Added a IStaticFileHostGenerator so its possible to replace our logic

* add docs and ensure the path is uniform with single slashes

---------

Co-authored-by: Bjarke Berg <mail@bergmania.dk>

src/Umbraco.Web.Common/Hosting/UmbracoStaticFilePathGenerator.cs

@@ -0,0 +1,45 @@
+using Microsoft.Extensions.Options;
+using Umbraco.Cms.Core.Configuration;
+using Umbraco.Cms.Core.Configuration.Models;
+using Umbraco.Cms.Core.Hosting;
+using Umbraco.Cms.Core.WebAssets;
+using Umbraco.Cms.Web.Common.Hosting;
+using Umbraco.Extensions;
+
+public class UmbracoStaticFilePathGenerator : IStaticFilePathGenerator
+{
+    private string? _backofficeAssetsPath;
+    private readonly IHostingEnvironment _hostingEnvironment;
+    private readonly IUmbracoVersion _umbracoVersion;
+    private readonly IRuntimeMinifier _runtimeMinifier;
+    private readonly IOptions<GlobalSettings> _globalSettings;
+
+    public UmbracoStaticFilePathGenerator(IHostingEnvironment hostingEnvironment, IUmbracoVersion umbracoVersion, IRuntimeMinifier runtimeMinifier, IOptions<GlobalSettings> globalSettings)
+    {
+        _hostingEnvironment = hostingEnvironment;
+        _umbracoVersion = umbracoVersion;
+        _runtimeMinifier = runtimeMinifier;
+        _globalSettings = globalSettings;
+    }
+
+    /// <summary>
+    ///     Get the virtual path for the Backoffice assets coming from the Umbraco.Cms.StaticAssets RCL.
+    ///     The path will contain a generated SHA1 hash that is based on a number of parameters including the UmbracoVersion and runtime minifier.
+    /// </summary>
+    /// <example>/umbraco/backoffice/addf120b430021c36c232c99ef8d926aea2acd6b</example>
+    /// <see cref="UrlHelperExtensions.GetCacheBustHash"/>
+    public string BackofficeAssetsPath
+    {
+        get
+        {
+            if (_backofficeAssetsPath is null)
+            {
+                var umbracoHash = UrlHelperExtensions.GetCacheBustHash(_hostingEnvironment, _umbracoVersion, _runtimeMinifier);
+                var backOfficePath = _globalSettings.Value.GetBackOfficePath(_hostingEnvironment);
+                _backofficeAssetsPath = backOfficePath.EnsureEndsWith('/') + "backoffice/" + umbracoHash;
+            }
+
+            return _backofficeAssetsPath;
+        }
+    }
+}