Signer computes what to sign on its own

[jpraynaud]

Why

The signer needs to compute the next message to sign independently and without relying on the pending certificate of the aggregator.

What

Replace the usage of the Pending Certificate in the signer with its own computation of which is the next message to sign.

How

• Adapt the mechanism of the aggregator to compute the next signed entity type to sign
• Store the signed entity type signed persistently when it has been successfully (when signature is sent)
• Prune or keep last 2 epochs in the store?
• Remove all pending certificate code in the signer
• Deprecate the /pending-certificate route in OpenAPI