IF: Unification: Add block header extension

[arhag]

Depends on #1881 and #1941.

The block header extension should carry the new information needed for a proposal:

• last_qc_block_num: The block height of the most recent ancestor block that has a QC.
• is_last_qc_strong: This is a boolean that determines if the QC justification for the block referenced by last QC block height is strong or weak.
• new_finalizer_policy: The new finalizer policy proposed in the block (may be nullopt).
• new_proposer_policy: The new proposer policy proposed in the block (may be nullopt).

Example:

 struct instant_finality_extension {
    uint32_t last_qc_block_num {0};  ///< The block height of the most recent ancestor block that has a QC justification
    bool     is_last_qc_strong {false}; ///< whether the QC for the block referenced by last_qc_block_height is strong or weak.
    std::optional<finalizer_policy> new_finalizer_policy;
    std::optional<proposer_policy> new_proposer_policy;
 };

This should replace the extensions finalizer_policy_extension and proposal_info_extension.

Use the status of the pending_quorum_certificate to determine whether you have a valid QC and if so whether it is strong or weak. That determines the values of last_qc_block_num and is_last_qc_strong.

Use the most recent ancestor block that has a valid QC (whether the status is strong, weak_final, or weak_achieved) as the block referenced by last_qc_block_num.

The actual QC does not need to be included yet in a block extension. That will be handled in a separate issue.

[arhag]

Partially done, but to complete we must first wait until at least #2034 is complete and it would probably be best to wait until #2046 is also complete.

[greg7mdp]

I have finished this issue in my branch gh_2034.

[greg7mdp]

@linh2931 @arhag I believe we can close this issue as completed by #2077