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>