Missing identity property in Microsoft.Devices/IotHubs spec

The IoT Hub API supports the "identity" property, but it is not included in the spec (neither in the [preview spec](https://github.com/Azure/azure-rest-api-specs/blob/master/specification/iothub/resource-manager/Microsoft.Devices/preview/2020-07-10-preview/iothub.json) nor in the [latest stable](https://github.com/Azure/azure-rest-api-specs/blob/master/specification/iothub/resource-manager/Microsoft.Devices/stable/2020-08-01/iothub.json) one).

The following template has been generated by the Azure Portal and includes the "identity" property.
```
{
"$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.0",
"parameters": {
"iotHubName": {
"defaultValue": "iot-hub-with-system-assigned-identity-hdu48fh",
"type": "String"
}
},
"variables": {},
"resources": [
{
"type": "Microsoft.Devices/IotHubs",
"apiVersion": "2020-07-10-preview",
"name": "[parameters('iotHubName')]",
"location": "westeurope",
"sku": {
"name": "S1",
"tier": "Standard",
"capacity": 1
},
"identity": {
"type": "SystemAssigned"
},
"properties": {
"ipFilterRules": [],
"eventHubEndpoints": {
"events": {
"retentionTimeInDays": 7,
"partitionCount": 4
}
},
"storageEndpoints": {},
"messagingEndpoints": {
"fileNotifications": {
"lockDurationAsIso8601": "PT1M",
"ttlAsIso8601": "PT1H",
"maxDeliveryCount": 10
}
},
"enableFileUploadNotifications": false,
"cloudToDevice": {
"maxDeliveryCount": 10,
"defaultTtlAsIso8601": "PT1H",
"feedback": {
"lockDurationAsIso8601": "PT1M",
"ttlAsIso8601": "PT1H",
"maxDeliveryCount": 10
}
},
"features": "None"
}
}
]
}
```

Start by reading the linked preview file at specification/iothub/resource-manager/Microsoft.Devices/preview/2020-07-10-preview/iothub.json and the stable file at specification/iothub/resource-manager/Microsoft.Devices/stable/2020-08-01/iothub.json. Compare the IotHubs resource shape with the portal template and add the missing identity property to the applicable specs; done means both referenced API versions represent this supported property.

Written by the indexing model from the issue text.