List available subscriptions from the customer portal
As an API or CLI user, I want to list available subscriptions from the Portal.
Interfacing with the Portal should not require the user to provide username/password. Instead, the certificate from the manifest may be used.
Interfacing with the Portal should work with or without a proxy located between it and the Satellite server.
The ability to list subscriptions is controlled by the 'list subscriptions' permission.
The list includes sufficient details to allow the user to select the desired subscription (e.g. name, contract, start date, end date, available entitlements)
The list does not include subscriptions that have already been allocated. (Note: Provide a behavior that is consistent with the customer portal; therefore, if this is not how Portal behaves, it can be removed from acceptance)