Decentralization of signature orchestration

[rezabaram]

Why

In order to move further toward decentralization of the Mithril protocol where multiple aggregators will be able to operate, we need to decentralize the signature orchestration and make it happen independently on all the signers and aggregators.

What

Develop a signature orchestration mechanism which is compatible with a decentralized Mithril network. The implementation will work both with the current centralized setup for production, and with an experimental decentralized setup:

• The Mithril signer and aggregator nodes are able to compute independently the beacon which determines the messages to sign and certify.
• The Mithril aggregator does not advertise the pending certificates anymore (the pending certificate is deprecated and temporarily kept alive for legacy signer nodes and until sufficient adoption of the new version is reached)
• The Mithril aggregator buffers the individual signatures from signers until it has computed the associated beacon, and it will try to aggregate them thereafter.

How

• Signer retrieves registrations with epoch settings route #1897
• Aggregator exposes Cardano transactions signing configuration #1898
• Add message signed in signature HTTP messages #1899
• Aggregator buffers signatures for unknown open message #1900
• Reorganize signer dependencies #1906
• Retrieve custom signing configurations with epoch settings in signer #1923
• Aggregator advertises constant signing configurations for an epoch #1924
• Signer computes what to sign on its own #1925
• Refactor Certifier service to sign and send signatures in signer #1945
• Aggregator uses stored signing configurations when creating signed entity type #1961