Add listSubscriptions to resourceApi typings

[nturinski]

Implementation needs to be done in Resource Groups, which will be coming soon

[bwateratmsft]

Shouldn't the auth package be used instead?

[nturinski]

Maybe... Here's my scenario:

I need a way to list the filtered subscriptions to create my own SubscriptionListStep for a user to select their sub. This is happening in the Functions and Storage extension. Since I don't think we'd want the client extensions to directly consume the auth package, I was planning on having Resources handle listing the subs out to the clients.

Resources also controls the tenant/subscription filter settings, so I thought it made more sense for it to deliver the list to the clients.