[enhancement]: implement hotplug support for block devices

[DarkPhily]

Enhancement

I would like to implement a nativ cloud-init way to handle hotplug events for block devices.

We thought about to work around this cloud-init limitation, by using write_files in the vendor-data, but longterm it would be much nicer to have a native way.

I propose to extend event.py to include an EventScope.STORAGE Would you prefer to change cc_install_hotplug.py to include storage udev handling or should this be a separate file, like cc_install_block_device_hotplug.py?

Do you happen to know what files might need to be adjusted as well?

I'm happy about every input regarding this topic :)

[holmanb]

What use-cases do you have for this proposal?