feat: implement 'rewrite' storage middleware

This allows to rewrite 'URLFor' of the storage driver to use a specific
host/trim the base path.

It is different from the 'redirect' middleware, as it still calls the
storage driver URLFor.

For example, with Azure storage provider, this allows to transform the
SAS Azure Blob Storage URL into the URL compatible with Azure Front
Door.

Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

docs/content/storage-drivers/middleware/_index.md

@@ -0,0 +1,15 @@
+---
+description: Explains how to use storage middleware
+keywords: registry, on-prem, images, tags, repository, distribution, storage drivers, advanced
+title: Storage middleware
+---
+
+This document describes the registry storage middleware.
+
+## Provided middleware
+
+This storage driver package comes bundled with several middleware options:
+
+- cloudfront
+- redirect
+- [rewrite](rewrite): Partially rewrites the URL returned by the storage driver.

docs/content/storage-drivers/middleware/rewrite.md

@@ -0,0 +1,32 @@
+---
+description: Explains how to use the rewrite storage middleware
+keywords: registry, service, driver, images, storage, middleware, rewrite
+title: Rewrite middleware
+---
+
+A storage middleware which allows to rewrite the URL returned by the storage driver.
+
+For example, it can be used to rewrite the Blob Storage URL returned by the Azure Blob Storage driver to use Azure CDN.
+
+## Parameters
+
+* `scheme`: (optional): Rewrite the returned URL scheme (if set).
+* `host`: (optional): Rewrite the returned URL host (if set).
+* `trimpathprefix` (optional): Trim the prefix from the returned URL path (if set).
+
+## Example configuration
+
+```yaml
+storage:
+  azure:
+    accountname: "ACCOUNT_NAME"
+    accountkey: "******"
+    container: container-name
+middleware:
+  storage:
+    - name: rewrite
+      options:
+        scheme: https
+        host: example-cdn-endpoint.azurefd.net
+        trimpathprefix: /container-name
+```