Adds the `corso restore` command to the cli (which only displays
a help menu). And adds the `corso restore exchange`
command (which also only displays a help menu) to the cli.
Further functionality (like the complete flags) will come later.
* use config file for backup create
Now that corso stores its prior connection details in a config
file, that file should get used to retrieve storage details as needed.
credentials requriements surface in many places thorughout corso:
they can be sourced from many locations (envs, files, manually),
and used in many more (cli, repo, kopia). This usage could blossom
into all kinds of duplicate structs sharing similar info. The goal
of this change is to centralize where credentials are declared
and managed, and how they then cascade out to other packages.
