arti: Add hsc key subcommand, deprecate hsc get-key.

I am deprecating the old hsc get-key subcommand in favor of the new hsc key get subcommand. This is because I plan to implement the rest of the key management functionality (key deletion, rotation, etc.) as subcommands of the hsc key command. The alternative would be to add a new distinct top-level hsc rotate-key, hsc remove-key, etc. subcommand alongside the existing hsc get-key command (which IMO is less nice than the alternative I'm proposing).