Design and implement generic signing/verification entity services

[jpraynaud]

Issue

Design the interfaces and scope of entity services that will be implemented in the aggregator, signer and client nodes to handle different types of data. Each entity service should be able to compute a message given a custom beacon and provide that message to the signing engine. The signing engine is agnostic of the messages that are signed. The certificate chain should be modified to provide a certificate for each new message of each type of data, and link that certificate to the first certificate of the epoch (which will sign the Mithril stake distribution used in the next epoch)

To do

• Define the interface of the generic entity service for signing #847
• Define the interface of the generic entity service for verification #868
• Implement signable builder for Mithril Stake Distribution #851
• Implement signable builder for Full Immutables Snapshot #852
• Adapt runtime to use artifact builder service in aggregator #869
• Adapt runtime to use signable builder service in aggregator #853
• Adapt runtime to use signable builder service in signer #854
• Extend SignedEntityType to hold beacon or epoch #848
• Implement Tick service in aggregator #849
• Implement Certifier service in aggregator #850
• Implement artifact builder for Mithril Stake Distribution #870
• Implement artifact builder for Full Immutables Snapshot #871
• Enhance state machines Aggregator/Signer #933
• Cleanup the Multi-signer state
• Adapt the aggregator REST API pending certificate route to broadcast the signed entity type currently signed
• Adapt the aggregator REST API to list certificates #892
• Adapt the aggregator REST API to retrieve list/detail of signed entities by signed entity type #893
• Handle signed entity types in client #894
• Adapt end to end tests to handle new types of data #899
• Update architecture documentations for new types of data #898
• Adapt the explorer to handle new aggregator /artifact routes #927

Later

• Adapt the explorer to handle new aggregator /certificates route
• Decommission previous snapshot routes from the aggregator REST API

Future

• Implement the interface for the Cardano Stake Distribution signed entity type
• Make the signer compute its own open messages
• Decommission the pending certificate route (and use the epoch settings route for the protocol parameters and signer registrations broadcast)
• Implement multiple certificates creation at the same time (with parallel runtimes and adaptation of the register signatures routes)

Depends

Issue #779