Add an internals subcommand

[SamD2021]

Adding an internals subcommand to Lowell would make it easier to reach common functionality that doesn't make sense to be user-facing.
It could be something like:

lowell internals parse-bin

It should not be promoted to the user and should act in a similar way to bootc internals

[rajeshrah22]

What can this be used for?

Integration tests?

[SamD2021]

It could be used for utilities outside the scope of what we want to ship to users, like helper binaries, likely also integration tests.