Enable dry run mode for files-delete command

[svenmueller]

This would makes sense, e.g. if you could test the regular expression and see which files are affected before you use the same regular expression for the delete command

# before running the command
$ s3utils files delete -p <prefix> -r <regex>

# check what files will be removed
$ s3utils files delete -p <prefix> -r <regex> --dry-run

[emmenko]

:+1:

[svenmueller]

i think the files list command also makes sence (we already have the logic we only have to expose it)

[emmenko]

Sure, we can add that command as well. We just need to decide how we want to "provide" the list (I'll create a new ticket)