Add global `--dry-run` parameter

It would be useful to have an option like the git --dry-run that parses all args etc but doesn't run the command. Even better if it could output what it would do.

There are many fairly complex options to get right so a pre flight check is useful.

I guess if the commands always fully fail that is almost as good.

The issue names no files, tests, or entry points. Start by locating the AWS CLI's global argument parsing and command execution entry points, then clarify whether completion requires only parsing without execution or also reporting planned actions; verify that both behaviors are defined before implementation.

Written by the indexing model from the issue text.