Introduces a NewRestore method to the Repository struct.
cli/restore/exchange now generates a restoreOperation
and kicks off a Run() when `corso restore exchange` is called.
The restore operation still only accepts the placeholder
Target string slice.
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.
