account needs a way to list contracts / subscribtions with providers

[Gozala]

Right now provider/* protocol defines a how an account can get a provider and then add it to a specific space. However currently there is no way for the account to check what providers has it added to which spaces.

Given that some providers MAY be limited to e.g. one per account (which is currently the case) it is important for the application to be able to tell if it should create a space and arrange provider for it, or not. This is also amplified by the fact that application has no way of distinguishing where access to some space had been delegated by another user, making it difficult to decide if new namespace should be created.

It could optimistically attempt to get a store provider and conclude it has already used space provider if fails, but that does not seem ideal.

I think it would be a good idea to extend provider protocol or define a separate protocol that would allow account to:

1. List out subscriptions (contracts if you will) like { space, provider }.
2. Terminate subscription it no longer wishes to have.