NewTransport returns a POST-only DoH transport. `id` identifies this transport. `rawurl` is the DoH template in string form. `addrs` is a list of IP addresses to bootstrap dialers. `px` is the proxy provider, may be nil (eg for id == dnsx.Default)
(ctx context.Context, id, rawurl string, addrs []string, px ipn.ProxyProvider)
source not stored for this graph (policy: none)