Extra operators should be available on top level only (@sortBy)

[LastDragon-ru]

Makes no sense to have

order: [
    {random: yes},
    {field: {random: yes}},
],

Probably we can create an additional type for the root field (where order argument is defined) that will contain fields and extra operators. It will also allow to have property field with all fields and avoid name conflicts (current version will throw an error if the field with the same name as extra operator exists).