specify info capability

[Gozala]

On store/add we need an info capability so it can tell us about the state of the storage provider, whether it has enough capacity to hold data been added.

On provider/get and provider/add we need info capability to ensure that account has an active payment provider that we can bill.

Perhaps we need same info capability to render account page that reports how much storage is available across spaces, who provides them and what are the terms.

I'm starting to think it would make more sense to fold it into provider namespace e.g. provider/info as it returns info about providers that given did has.