az eventgrid event-subscription create command error

### Related command
az eventgrid system-topic event-subscription create
az eventgrid event-subscription create

### Extension name (the extension in question)
az eventgrid

### Description of issue (in as much detail as possible)

In Azure Portal, we can create an event subscription for system topic with enabling dead-lettering as shown below: ([doc reference](https://learn.microsoft.com/en-us/azure/event-grid/manage-event-delivery#azure-portal))


However, [az eventgrid system-topic event-subscription create](https://learn.microsoft.com/en-us/cli/azure/eventgrid/system-topic/event-subscription?view=azure-cli-latest#az-eventgrid-system-topic-event-subscription-create) doesn't have parameters `deadletter-identity` and `deadletter-identity-endpoint` in CLI and running results in `unrecognized arguments: --deadletter-identity-endpoint /subscriptions/xxx/resourceGroups/xxx/providers/Microsoft.Storage/storageAccounts/xxx/blobServices/default/containers/egcontainer --deadletter-identity systemassigned` error.

So followed [az eventgrid topic event-subscription create](https://learn.microsoft.com/en-us/cli/azure/eventgrid/topic/event-subscription?view=azure-cli-latest#az-eventgrid-topic-event-subscription-create) doc and ran the following command and it also resulted in error.
**Command:**
az eventgrid event-subscription create --name xxx --source-resource-id "/subscriptions/xxx/resourceGroups/xxx/providers/Microsoft.EventGrid/systemTopics/xxx" --endpoint-type storagequeue --endpoint "/subscriptions/xxx/resourceGroups/xxx/providers/Microsoft.Storage/storageAccounts/xxx/queueservices/default/queues/xxx" --deadletter-identity-endpoint "/subscriptions/xxx/resourceGroups/xxx/providers/Microsoft.Storage/storageAccounts/xxx/blobServices/default/containers/egcontainer" --deadletter-identity systemassigned

**Output:**
Code: InvalidRequest
Message: This event subscription operation is not supported using this API call. Please use the proper API and try again.

Any plan for supporting deadletter-identity for system topic? These parameters are available in both [ARM template](https://learn.microsoft.com/en-us/azure/templates/microsoft.eventgrid/2021-10-15-preview/topics/eventsubscriptions?pivots=deployment-language-arm-template) as well as [Rest API](https://learn.microsoft.com/en-us/rest/api/eventgrid/controlplane-version2022-06-15/system-topic-event-subscriptions/create-or-update?tabs=HTTP).