List operators
Returns a collection of existing operators.
It will return an array of items with following properties:
* `id`: identifier of this operator
* `name`: the official name of the operator
* `name_and_country`: Name / Country (for easy access in the UI)
* `country`: country object with the nested country `id`
* `mnc`: Array of Mobile Network Codes (MNCs)
* `tapcode`: Array of TAP Codes