Update CLI to support creating or updating a user assigned Managed Identity for Event Grid

**Related command**
az eventgrid domain create/update
az eventgrid event-subscription create/update

**Is your feature request related to a problem? Please describe.**
I'd like to use a user assigned managed identity with a "Storage Blob Data Contributor" role to enable event grid to write to an azure storage account for dead-lettering for event grid subscriptions

**Describe the solution you'd like**
update the Azure CLI to support assigning user assigned managed identity to event grid domains and topics

**Describe alternatives you've considered**
using systemassigned instead of userassigned identities, but I'd rather the identity not be destroyed with the resource it's tied to

**Additional context**
This doc: https://learn.microsoft.com/en-us/azure/event-grid/enable-identity-custom-topics-domains?tabs=cli#enable-identity-when-creating-a-topic-or-domain, under the Azure CLI tab specifically says "Currently, Azure CLI doesn't support assigning a user-assigned identity to a topic or a domain".