Provide interface for retrieving SD from external source in Aggregator/Signer

[jpraynaud]

The Mithril Aggregator/Signer must use a common interface (aka a chain observer) in order to be able to gather & use information from the Cardano chain. This issue aims at creating this interface and wiring it in the nodes, as well as providing a fake implementation of it. This will also enables the network to use previous stake distribution and verification keys (based on the previous epoch)

Tasks to do:

• Add a ChainObserver trait in mithril-common
• Add a FakeObserver that implements a ChainObserver in mithril-common
• Use a FakeObserver in Mithril Aggregator
• Use a FakeObserver in Mithril Signer
• Implement a real -1 offset on the epoch used for retrieving the verification keys and the stake distribution

More information in #253