[API] Add callback to notify for broker/account disconnection/reconnection

Hi, in cases where broker/account is disconnected and maybe reconnected, we want a way to notify user.

Imagine we implement a chart strategy and we leave the computer, we want a way to notify ourselves if the broker connection has any issue, so we can FLAT the position with our mobile, or something. We can call a Telegram bot or similar strategy, but we need a reliable way to detect this scenarios and take action to prevent possible loss.

Thanks.