Produce valid certificate chain for several epochs on Devnet

[jpraynaud]

Issue

• Some epochs are not signed (with empty signers list in the Pending Certificate)
• The certificate chain is broken in flaky fashion after several epochs with an AVKMismatch Error when the stake distribution is updated
• The Signers get Lottery for this epoch was lost error from time to time when the stake distribution is updated

Tasks

• "Some epochs are not signed" is fixed with Add state machine runtime Signer #317
• Return an error when Aggregator detects an Epoch gap between the last certificate and the current Epoch (See Fix Certificate Chain append in Aggregator #418)
• Fine tune test end to end to reduce flakiness (see Attempt fix CI flakiness #419)
• Add Certificate Chain integration tests (see Certificate chain integration test for Aggregator #424)

[jpraynaud]

I close this issue as the remaining fixes/development on the Certificate Chain are embedded in #364