[enhancement]: implement hotplug support for block devices

# 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](https://github.com/canonical/cloud-init/blob/main/cloudinit/event.py) to include an EventScope.STORAGE
Would you prefer to change [cc_install_hotplug.py](https://github.com/canonical/cloud-init/blob/main/cloudinit/config/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 :)