Required expiry flag and versioning

[SequeI]

Right now when a user wishes to use tuftool update they are required to update all metadata files, while it technically should be possible to only update timestamp+snapshot as an example.

Also, right now a user can provide existing version numbers and overwrite the metadata files created and a well behaving TUF client will consider this an error. Versions should be calculated automatically in the background to remove potential issues

Maybe also introduce a --force flag in the future to give a user the ability if they wish to

[mgsharm]

Hi @SequeI , thanks for opening the issue. We will take a look at the feature request and get back to you. Thanks!