main: provide `--preview` to toggle preview state

Certain image types have the notion of being in a preview state. Also
called "final", "supported", or "prerelease".

In `images`'s distribution definitions we define this flag per version
of a distribution. This means that we need to quickly do releases to
update this value before a release candidate is built (release
candidates are built as "supported").

When pungi schedules composes it knows the supported state of the
compose. Thus it is a good idea to let the preview state of a
distro be overridden by the `pungi` -> `koji` -> `image-builder`
chain.

Let's include a `--preview` argument. When the option is not passed at
all we let `images` determine the preview state (based on its
distribution definitions). When the option is passed we pass along its
value to `images` where it is then used instead of the default.

I've kept the option hidden here because it really only should be used
by build systems or advanced users (we might document its use) and not
confuse normal users; who would usually receive new distribution
definitions with the next `image-builder` release which contain the
correct preview state, though it will lag a bit in time.

€ ./image-builder manifest --distro fedora-44 everything-network-installer 2>/dev/null | rg final
                        "final": false,
€ ./image-builder manifest --distro fedora-44 --preview=true everything-network-installer 2>/dev/null | rg final
                        "final": false,
€ ./image-builder manifest --distro fedora-44 --preview=false everything-network-installer 2>/dev/null | rg final
                        "final": true,
€ ./image-builder manifest --distro fedora-44 --preview everything-network-installer 2>/dev/null | rg final 
                        "final": false,