Add dynamic completion scaffolding to the clan `cli`.
Also add a dynamic completion mechanism for machines for commands that
have machines as their sole argument.
More intricate dynamic completions will be implemented in follow up
PR's.
Injecting nixos configuration and potentially overriding settings a user
made and can cause surprises.
In most cases, users want to just make these option part of their NixOS
configuration and by having the rest in the command line
we make it more explicit what other configuration is being applied.
