serve: pass the client IP address to the auth proxy - fixes #4499

The auth proxy was only given the user and their password or public
key, so a proxy program had no way to restrict logins to particular
networks, or to record where an authentication attempt came from.

The JSON sent to the program now has a client_ip key holding the bare
IP the client connected from, with the port stripped so IPv6 arrives
as 2001:db8::1 rather than [2001:db8::1]:52344. An IPv4-mapped IPv6
address is reported as plain IPv4 so that a client arriving over a
dual-stack listener still matches IPv4 networks. The key is omitted
when the client has no IP address.

The IP is also mixed into the backend cache key. That is needed as the
program is only run on a cache miss, so a client from a
non-allowlisted address presenting valid credentials within the 5
minute cache lifetime would get a cache hit and be let in without the
program being consulted at all.

lib/http/middleware.go

@@ -142,7 +142,7 @@ func MiddlewareAuthCustom(fn CustomAuthFn, realm string, userFromContext bool) M
 				return
 			}
 
-			value, err := fn(user, pass)
+			value, err := fn(r, user, pass)
 			if err != nil {
 				fs.Infof(r.URL.Path, "%s: Auth failed from %s: %v", r.RemoteAddr, user, err)
 				goauth.NewBasicAuthenticator(realm, func(user, realm string) string { return "" }).RequireAuth(w, r) //Reuse BasicAuth error reporting