Register multiple keys for alerts and private notifications
Currently only a single key may be registered for [alerts](https://github.com/haveno-dex/haveno/blob/01dd6a8af96a84c841efc5e192ff54e418367174/core/src/main/java/haveno/core/alert/AlertManager.java#L96) and [private notifications](https://github.com/haveno-dex/haveno/blob/01dd6a8af96a84c841efc5e192ff54e418367174/core/src/main/java/haveno/core/alert/PrivateNotificationManager.java#L96).

This issue requests supporting multiple keys.