set the africastalking POST route using env

.env.example

@@ -16,3 +16,6 @@ CREATE_ACCOUNT_URL=http://localhost:5003/api/v2/account/create
 TRACK_STATUS_URL=https://custodial.sarafu.africa/api/track/
 BALANCE_URL=https://custodial.sarafu.africa/api/account/status/
 TRACK_URL=http://localhost:5003/api/v2/account/status
+
+#AfricasTalking USSD POST endpoint
+AT_ENDPOINT=/ussd/africastalking

cmd/africastalking/main.go

@@ -197,9 +197,13 @@ func main() {
 	rp := &atRequestParser{}
 	bsh := handlers.NewBaseSessionHandler(cfg, rs, stateStore, userdataStore, rp, hl)
 	sh := httpserver.NewATSessionHandler(bsh)
+
+	mux := http.NewServeMux()
+	mux.Handle(initializers.GetEnv("AT_ENDPOINT", "/"), sh)
+
 	s := &http.Server{
 		Addr:    fmt.Sprintf("%s:%s", host, strconv.Itoa(int(port))),
-		Handler: sh,
+		Handler: mux,
 	}
 	s.RegisterOnShutdown(sh.Shutdown)