service: rework Subscription

[vgonkivs]

Our current Subscription method, which appears to allow users to subscribe to a specific proof type, is actually providing access to a particular pub-sub topic. This is not in line with our intended functionality, as pub-sub interactions should be restricted to internal usage only. To address this issue, I propose that we discontinue the option for users to interact with the pub-sub mechanism. Instead, we should offer a subscription to the datastore where proofs are stored. This subscription would return an iterator, enabling users to navigate between different proofs for a single proof type. Additionally, implementing this change would grant us the capability to enable subscriptions even before the service starts, a feature that is currently unavailable.

Moreover, this new approach would allow us to access proofs one by one, which is especially beneficial when dealing with a large number of proofs. It simplifies the process of finding a specific proof among many.