tuftool: Allow updating only a subset of metadata files

[bkabrda]

Hi folks 👋,
I noticed that when running tuftool update, it is always necessary to provide arguments to update timestamp, snapshot and targets. In reality, it should be possible to update only timestamp+snapshot or only timestamp. I think this should be fairly easy to implement and I'd be willing to make the contribution assuming you all agree that this feature makes sense (also I would need to learn Rust first, so it might take some time, but that's another story 😄 ).

Please let me know what you think, thanks!