Move build from positional args to named.

[supakeen]

It'd be nicer if the image-builder build subcommand takes named arguments instead of positional arguments. Currently it looks like this:

image-builder build --distro fedora-41 minimal-raw blueprint.toml

I propose:

image-builder build --distro fedora-41 --type minimal-raw --blueprint blueprint.toml

This would allow for multi-type builds (a very strong advantage of the osbuild system):

image-builder build --distro fedora-41 --type iso --type qcow2 --blueprint blueprint.toml

And would allow us to in the future have other inputs except the built-in image types such as a custom YAML file for custom images.

[mvo5]

Fwiw, I like that it's more "uniform" this way, i.e. to me it feels a bit unbalanced that we currently have "--distro centos-9" but type is just an positional argument and has no --type. But then its a bit more typing for our users.

We should probably have a brainstorm again in the new year.