[POC] (C11N) lib/base: add "redirect" config

WIP: Tests need to be written

The "redirects" config allows configuring redirects from paths to routes.

Example config:

   "redirects" => { "^/path/regexp" => "an.apps.controller.route.locator" }

Use case:

* An administrator can configure another app as target for certain paths.

Differentiation from Apache redirects:

* this allows configuring redirects in one place and avoids spreading
  the configuration in multiple places.

* the target route locator is more expressive/easier to trace than a
  path

config/config.redirects-sample.php

@@ -0,0 +1,13 @@
+<?php
+$CONFIG = [
+  'redirects' => [
+    // Request path without /index.php/ maps to a controller path in the form
+    // <app name>.<controller name>.<handler>.
+
+    // - For a FooController.php the controller name is "foo" (lowercase)
+    // - A handler would be a method in FooController that was annotated with
+    //   - either #[FrontpageRoute] attribute
+    //   - or configured in routes.php
+    '^\/settings' => 'acmesettings.page.index'
+  ],
+];

lib/base.php

@@ -1047,7 +1047,27 @@ public static function handleRequest(): void {
 					OC_App::loadApps(['filesystem', 'logging']);
 					OC_App::loadApps();
 				}
-				Server::get(\OC\Route\Router::class)->match($request->getRawPathInfo());
+				$requestPath = $request->getRawPathInfo();
+				$redirects = $systemConfig->getValue('redirects', []);
+
+				if ($redirects) {
+					foreach ($redirects as $fromPattern => $toLocator) {
+						if (!preg_match('/' . $fromPattern . '/', $requestPath)) {
+							continue;
+						}
+
+						try {
+							$targetLocation = Server::get(IURLGenerator::class)->linkToRouteAbsolute($toLocator);
+							header('Location: ' . $targetLocation);
+							return;
+						} catch (\Exception) {
+							// In case of container exceptions or
+							// route not found exceptions we proceed as usual.
+						}
+					}
+				}
+
+				Server::get(\OC\Route\Router::class)->match($requestPath);
 				return;
 			} catch (Symfony\Component\Routing\Exception\ResourceNotFoundException $e) {
 				//header('HTTP/1.0 404 Not Found');