main.rs: Refactor the way command line fields are defined

[bgartzi]

Check if it is worth it to duplicate some of the command line interface argument field definition for the sake of a clearer main.rs file.

See https://github.com/confidential-clusters/compute-pcrs/pull/7/files/9293762450f0d53896823a14f3b715cb7694434a#r2257254571 for more information.