incus-simplestreams add: add --alias flag

[melato]

incus-simplestreams-add generates an alias for the image using {os}/{release}/{variant}.

This is a feature request to add an "--alias " flag to the command, to override this default alias.

Rationale: I want to start using simplestreams to share my images across servers, and I want to keep the image aliases that I've been using already, instead of changing them to the above format in all my scripts. I also don't want to have slashes in the aliases, because I generate and save each image in a directory on disk, and use the directory name as the image alias. Being able to specify my own alias will make it easier to start using a simplestreams server for my images, which will facilitate sharing images.