proxy: User ReverseProxy.Rewrite instead of Director

With Go 1.20 the "Rewrite" hook for ReverseProxy was introduced to
supersede of the "Director" hook (see:
https://github.com/golang/go/commit/a55793835f16d0242be18aff4ec0bd13494175bd)

The Rewrite hooks allows for better separation between the incoming and
outgoing request. In particular it makes it pretty easy to set the
correct X-Forwarded-* Headers on the outgoing request.
The need for using "Rewrite" came up when trying to embed
authelia. It uses the X-Forwarded-Host and X-Forwared-Proto headers to
e.g. compute the correct return values for the various endpoints in
.well-known/openid-configuration.

services/proxy/pkg/proxy/proxy.go

@@ -36,9 +36,9 @@ func NewMultiHostReverseProxy(opts ...Option) (*MultiHostReverseProxy, error) {
 		config:    options.Config,
 	}
 
-	rp.Director = func(r *http.Request) {
-		ri := router.ContextRoutingInfo(r.Context())
-		ri.Director()(r)
+	rp.Rewrite = func(r *httputil.ProxyRequest) {
+		ri := router.ContextRoutingInfo(r.In.Context())
+		ri.Rewrite()(r)
 	}
 
 	tlsConf := &tls.Config{