Support an IVC follower aggregator joining the network at any time

[jpraynaud]

Why

An IVC follower aggregator can currently only produce certificates on a chain that has been IVC since its genesis: the recursion is seeded from the parent certificate, and the synchronizer mirrors the leader's certificates whatever their proof system. Behind a Concatenation leader, an IVC follower therefore never produces a certificate, and the only bootstrap window (the epoch right after a genesis) can be missed depending on synchronization timing. An analysis has been led in #3389.

What

Make a follower aggregator able to join the network and start producing certificates at any time, for the following topologies:

#	Follower	Leader for signers	Source for certificates	Certification start
1	IvcSnark	Concatenation leader	Concatenation leader	At the epoch after the next re-genesis, once the follower is warmed up
2	IvcSnark	Concatenation leader	Caught-up IVC follower aggregator	At any epoch, by inheriting the rolling state from the synchronized parent certificate
3	Concatenation	Concatenation leader	Concatenation leader	Unchanged current behavior (warm follower: next epoch; cold follower: two epochs after genesis)
4	Concatenation	Concatenation leader	Caught-up Concatenation follower aggregator	Same as case 3, with the certificate chain synchronized from the intermediate follower

How

• Add an integration test checking that a warm follower signs at the epoch immediately after the genesis even when it synchronizes the genesis certificate only at that epoch
• Fetch the parent of a certificate that certifies the full certificate chain, as its verification stops early, so the synchronized chain stays contiguous down to the genesis certificate
• Support following one aggregator for the signer registrations and, optionally when specified, another one for the certificate chain (which defaults to the aforementioned)
  • Add the optional certificate chain aggregator endpoint to the configuration, the command line and the documentation
  • Instantiate a second aggregator client on that endpoint for the certificate chain synchronizer and its certificate verifier
  • Add the endpoint to the infrastructure (Terraform variable, aggregator environment, follower docker compose override)
  • Add the endpoint to the deployment workflows
• Adapt the end-to-end test to support a follower of follower topology for the certificate chain