Unable to set Event Grid to use Managed Identity through CLI

**Related command**

az eventgrid system-topic event-subscription update....

**Is your feature request related to a problem? Please describe.**

We have a fully automated process to build out additional event subscriptions under our eventgrid system topics, however we always have to perform the final step manually. Through the portal the process is Event Grid > System Topics > Select System Topic > Event Subscription > Select Event Subscription > Additional Features > Set Managed Identity Type to “System Assigned”

**Describe the solution you'd like**

I would like if we could do this through AZ CLI to match the rest of our playbook.

**Describe alternatives you've considered**

**Additional context**