consensus: Include ratification signature and bitset in block certificate

[goshawk-3]

Describe the bug
As proved in #1020 , a single agreement message is completely enough for creating a block with valid certificate that is accepted by the network.

We should consider adding ratification aggregate signature together with a bitset of voters in the block certificate.

pub struct Certificate {
    pub first_reduction: StepVotes,
    pub second_reduction: StepVotes,
    pub ratification: StepVotes,  /// New aggregate signagure
}

pub struct StepVotes {
    pub bitset: u64,
    pub aggregate_signature: Signature,
}

Additional context
If this is issue is approved for implementation, we need first to address #1026 otherwise aggregate_signature and bitset from 2nd reduction can be reused.

[fed-franz]

This is an alternative solution to the "accepting a block from a single Agreement" proposal, right?
It should be deprecated if accepting the DIP

[herr-seppia]

Resolved in #1346