Cannot delete user assigned identity

[xuzhang3]

Resources that support user assigned identities cannot delete existing identities. From the API perspective, we need to set the value in the identity key:value map to null to delete the identity, but object in SDK is not pointer, we cannot set it to nil to tell the service delete the identity.

Affected resources: azurerm_redis_cache, azurerm_container_registry etc. that support user-assigned identities

SDK code: https://github.com/hashicorp/go-azure-helpers/blob/main/resourcemanager/identity/system_and_user_assigned_map.go#L20

Current SDK request:

{
    "identity": {
        "type": "UserAssigned",
        "userAssignedIdentities": {
            "/subscriptions/xxx/resourceGroups/xxx/providers/Microsoft.ManagedIdentity/userAssignedIdentities/identity": {}
        }
    }
}

Expected Delete Request:

{
    "identity": {
        "type": "UserAssigned",
        "userAssignedIdentities": {
            "/subscriptions/xxx/resourceGroups/xxx/providers/Microsoft.ManagedIdentity/userAssignedIdentities/identity": null
        }
    }
}