Get the asset class × threat class coverage of the risk register.

{  "assetClasses": [    {      "id": "string",      "name": "string",      "assets": 0    }  ],  "threatClasses": [    {      "id": "string",      "name": "string"    }  ],  "cells": [    {      "assetClassId": "string",      "threatClassId": "string",      "expected": true,      "risks": 0,      "unrated": 0,      "level": 0    }  ]}