command_name: A string. Required for all IDL commands. Matches the command's canonical name returned by Command::getName(). For consistency, add it to all IDL commands, not just those in API Version 1.

Today, a command's name is the same as its element name in YAML, but in the future we may have several specifications for the same command, with different API versions. Since YAML elements must have unique names, we add the "command_name" field.