Signer retrieves registrations with epoch settings route

[jpraynaud]

Why

In order to decentralize further the protocol, we want to get rid of the centralized orchestration of the signatures which is done with the pending certificate mechanism. The signer registrations for the current and the next epoch should be accessible from a different route than the /certificate-pending route and this should be the source of the signer registrations for the current and the next epoch for all the signers.

What

Give access to the signer registrations for the current and the next epoch in the /epoch-settings route and deprecate the /pending-certificate route.

How

• Deprecate signers and next_signers fields in the /certificate-pending route in OpenAPI (also protocol and next_protocol)
• Store new data current_signers and next_signers in the EpochService and create current_signers and next_signers function to access them
• Use the current_signers and next_signers of the EpochService to add the signers and next_signers in the /epoch-settings route of the aggregator
• Record the signers registrations for the the current and the next epoch in a new EpochService in the signer during the transition from Unregistered to Registered in the signer
• Use the signers registrations for the the current and the next epoch stored in the EpochService and use them to sign the messages during the transition from Registered to Signed in the signer