List the subscriptions belong to management-groups

**Describe the problem **
Seems currently there is no way to list the subscriptions belonging to a management group.
- you can list the mgmt-groups
- you can list subscriptions, but the details dont include mgmt-group membership
- you can create/update mgmt-groups
- you can assign subs to mgmt-groups

**Describe the solution you'd like**
It be great if there was a "**az account management-group subscriptions show**" command
e.g.
az account management-group subscriptions show -n

**Describe alternatives you've considered**
There is an api call for this :
GET https://management.azure.com/providers/Microsoft.Management/managementGroups/{groupId}/subscriptions?api-version=2020-05-01