Update snapshot functionality based on real world usage

[traversaro]

• Do not load snapshot files from command line, but rather via the rosdistro_snapshot yaml argument.
• In vinca snapshot, by the default save snapshot with names rosdistro_snapshot.yaml, to avoid confusion with other "snapshot", like the pixi lockfiles or the conda-forge pinning files.
• Add as a comment the timestamp at which a given snapshot was taken

[Tobias-Fischer]

Thanks @traversaro that's great :)