Remove Operational Certificate circular dependency in KES signature

[jpraynaud]

Why

The chain observer trait which computes the KES period depends for legacy reasons of the Operational Certificate. However, the Operational Certificate is not necessary to compute the KES period and its usage is due to the implementation done for the Cardano CLI which requires the Operational Certificate to retrieve the KES period. The problem is that the KES signer returns the KES signature and the Operational Certificate when the signature is computed, and that it uses the KES period as an input of the computation, which is itself computed with the Operational Certificate.

What

Remove the circular dependency on the Operational Certificate.

How

• Remove the OperationalCertificate input in the get_current_kes_period function of the ChainObserver trait
• Adapt the implementation of the ChainObserver trait for the Cardano CLI
• Adapt the other implementations of the ChainObserver trait
• Run an aggregator stress test