Cardano Stake Distribution Certification

[jpraynaud]

Issue

Sign a new type of data, the Cardano Stake Distribution for the previous epoch

To do

• [ ] Add a new signed entity type (type + database migration)
• [ ] Create a new era that will activate this new type (see #941)
• [ ] Add a new Signable builder for the type
• [ ] Add a new Artifact builder for the type
• [ ] Add new routes in the aggregator REST API
• [ ] Add new sub command in the client to download the artifacts
• [ ] Update documentation
• [ ] TBD