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_view.go

@@ -896,12 +896,22 @@ func (v HTTPHandlerView) AcceptAppCaps() views.Slice[tailcfg.PeerCapability] {
 	return views.SliceOf(v.ж.AcceptAppCaps)
 }
 
+// Redirect, if not empty, is the target URL to redirect requests to.
+// By default, we redirect with HTTP 302 (Found) status.
+// If Redirect starts with '<httpcode>:', then we use that status instead.
+//
+// The target URL supports the following expansion variables:
+//   - ${HOST}: replaced with the request's Host header value
+//   - ${REQUEST_URI}: replaced with the request's full URI (path and query string)
+func (v HTTPHandlerView) Redirect() string { return v.ж.Redirect }
+
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
 var _HTTPHandlerViewNeedsRegeneration = HTTPHandler(struct {
 	Path          string
 	Proxy         string
 	Text          string
 	AcceptAppCaps []tailcfg.PeerCapability
+	Redirect      string
 }{})
 
 // View returns a read-only view of WebServerConfig.