Get the list of the Global Administrator role

**Problem**
The `az cli role assignment list` command do not allow to get Directory Roles affected to AD users such as "Global reader" or "Global Administrator". (see https://docs.microsoft.com/en-us/azure/active-directory/users-groups-roles/directory-assign-admin-roles)

**The expected solution**
Get the members of any Directory Roles with azure-cli

**The current alternative**
It seems only PowerShell could do it currently.