[RFE] Add support for multiple reboot windows

[steled]

Current situation

Currently it is only possible to have one reboot window.

Impact

This means I can only schedule reboots for one specific day or every day.

Ideal future situation

It would be great to setup multiple reboot windows, for example:

• Mon to Fri from 09:00 to 17:00 o'clock (office hours)

Implementation options

/bin/update-operator \
 --reboot-window-start="Mon, Tue, Wed, Thu, Fri 09:00" \
 --reboot-window-length=8h

[invidian]

Thanks for opening separate issue. More context here: #193.

If anyone wants to try implementing it, please go ahead. Existing implementation has plenty of tests, so it should be relatively easy to write new tests and extend it.