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

[mgmccarthy]

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".

[yonzhan]

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