upgrade notification

[axot]

Is your feature request related to a problem? Please describe.
The automatic upgrade mechanism is awesome, after few time upgrades,
I found sometime my workload will get corrupt.
Therefore, I'm start think about if there is a notification message to remind me
check the workload works will be very important.

Describe the solution you'd like
If user enabled auto upgrade via k3os.io/upgrade: true.
Send a notification message eg: email, before performing a upgrade,
and once the upgrade finished, send a summary notification also.

Describe alternatives you've considered

Additional context

[dweomer]

As upgrades are implemented leveraging SUC, this suggestion would be more appropriate there: https://github.com/rancher/system-upgrade-controller/issues