Support user assigned identities in Grafana monitor integrations

It appears as if the Azure CLI AMG extension does not have support for user assigned identities, because with the following command I get the following error: "The Grafana instance does not have a managed identity.".

```
az grafana integrations monitor add --name amg-example --resource-group rg-grafana --monitor-name monitor-workspace-example --monitor-resource-group-name rg-monitor-workspace --skip-role-assignments
```

az grafana show output, replaced the guids with empty guids:
```json
{
...,
"identity": {
"type": "UserAssigned",
"userAssignedIdentities": {
"/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/rg-grafana/providers/Microsoft.ManagedIdentity/userAssignedIdentities/mi-amg-example": {
"clientId": "00000000-0000-0000-0000-000000000000",
"principalId": "00000000-0000-0000-0000-000000000000"
}
}
},
...
}
```

https://github.com/Azure/azure-cli-extensions/blob/682e555778840fe43c2cd86c2e8bf7d205cde9d9/src/amg/azext_amg/integrations.py#L24-L26