Allow trial licenses to be assigned by an administrator

[amandakay]

We currently disable AdHocSubscriptions at the tenant level.  Because of this limitation, our users are unable to trial the PVA ability.  We are able to assign trial licenses through the admin portal for Power Apps premium licenses, due to the fact that it is assigned to a tenant, and it would be great if we could allow our users the ability to explore PVA in the same way.