ipn: add support for HTTP Redirects (#17594)

Adds a new Redirect field to HTTPHandler for serving HTTP redirects
from the Tailscale serve config. The redirect URL supports template
variables ${HOST} and ${REQUEST_URI} that are resolved per request.

By default, it redirects using HTTP Status 302 (Found). For another
redirect status, like 301 - Moved Permanently, pass the HTTP status
code followed by ':' on Redirect, like: "301:https://tailscale.com"

Updates #11252
Updates #11330

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

ipn/ipn_clone.go

@@ -242,6 +242,7 @@ var _HTTPHandlerCloneNeedsRegeneration = HTTPHandler(struct {
 	Proxy         string
 	Text          string
 	AcceptAppCaps []tailcfg.PeerCapability
+	Redirect      string
 }{})
 
 // Clone makes a deep copy of WebServerConfig.