Serve static files relative to a given root.
The path to relative to the root is expected to be associated to the 'path' key in the routing context.
The path can be local or remote given that GVFS can be used.
The 'ETag' header is obtained from GLib.FileAttribute.ETAG_VALUE.
If the file is not found, the request is delegated to the next middleware.
If the file is not readable, a '403 Forbidden' is raised.
| root |
path from which resources are resolved |
| serve_flags |
flags for serving the resources |