Add a ClientConnected callback in the snowflake proxy library

IPtProxy has added callback in their snowflake patch for when a client has connected to a running snowflake proxy. This can facilitate UX features similar to what we have in the Snowflake Web Extension. See for example, Orbot's provided callback:

() -> {
    Prefs.addSnowflakeServed();
    if (!Prefs.showSnowflakeProxyMessage()) return;
    var message = String.format(getString(R.string.snowflake_proxy_client_connected_msg), ONION_EMOJI, ONION_EMOJI);
    new Handler(getMainLooper()).post(() -> Toast.makeText(getApplicationContext(), message, Toast.LENGTH_LONG).show());
}