Use managed identity to command creates an event subscription for an event grid system topic

[kmadof]

Related command
This is possible to use Managed Identity for custom topics like this

storageid=$(az storage account show --name demoStorage --resource-group gridResourceGroup --query id --output tsv)
deadletterendpoint="$storageid/blobServices/default/containers/<BLOB CONTAINER NAME>"

az eventgrid event-subscription create  
    --source-resource-id /subscriptions/$subid/resourceGroups/$rg/providers/Microsoft.EventGrid/topics/$topicname 
    --delivery-identity-endpoint-type servicebusqueue
    --delivery-identity systemassigned 
    --delivery-identity-endpoint $queueid
    --deadletter-identity-endpoint $deadletterendpoint 
    --deadletter-identity systemassigned 
    -n $sb_esnameq

But based on the documentation this is not possible for system topics.

az eventgrid system-topic event-subscription create --name
                                                    --resource-group
                                                    --system-topic-name
                                                    [--aad-tenant-id]
                                                    [--advanced-filter]
                                                    [--azure-active-directory-application-id-or-uri]
                                                    [--deadletter-endpoint]
                                                    [--delivery-attribute-mapping]
                                                    [--enable-advanced-filtering-on-arrays {false, true}]
                                                    [--endpoint]
                                                    [--endpoint-type {azurefunction, eventhub, hybridconnection, servicebusqueue, servicebustopic, storagequeue, webhook}]
                                                    [--event-delivery-schema {cloudeventschemav1_0, custominputschema, eventgridschema}]
                                                    [--event-ttl]
                                                    [--expiration-date]
                                                    [--included-event-types]
                                                    [--labels]
                                                    [--max-delivery-attempts]
                                                    [--max-events-per-batch]
                                                    [--preferred-batch-size-in-kilobytes]
                                                    [--qttl]
                                                    [--subject-begins-with]
                                                    [--subject-case-sensitive {false, true}]
                                                    [--subject-ends-with]

Is your feature request related to a problem? Please describe.
This is not possible to use managed identity for system topics.

Describe the solution you'd like
I would like to have parameters like

    --delivery-identity-endpoint-type servicebusqueue
    --delivery-identity systemassigned 
    --delivery-identity-endpoint $queueid
    --deadletter-identity-endpoint $deadletterendpoint 
    --deadletter-identity systemassigned 

for system topics.

Describe alternatives you've considered
NA

Additional context
NA

[yonzhan]

Thank you for opening this issue, we will look into it.