Checkpoint data signing

[Stefan-Ethernal]

Description

This PR introduces new fields to the Header.Extra, which are required for checkpointing mechanism and are part of consensus protocol.

Checkpoint data consists of:

• BlockRound - round for which we are building block at given height,
• EpochNumber - denoting current epoch,
• CurrentValidatorsHash - hash of the current validator set (for the current epoch),
• NextValidatorsHash - hash of the next validator set (applicable for next epoch, namely epoch ending blocks which contain such information),
• EventRoot - merkle root hash of all exit events triggered by the appropriate smart contract.

Other than that, checkpoint is described with BlockNumber, BlockHash and ChainID, but we don't write those information into the blockchain, as it would be redundant.

Checkpoint data hash is in the following format:

keccak256(abi.encode(uint256 chainId, uint256 blockNumber, bytes32 blockHash, uint256 blockRound, 
   uint256 epochNumber, bytes32 currentValidatorsHash, bytes32 nextValidatorsHash, bytes32 eventRoot))

Current validators and next validators hash is in the following format:

keccak256(abi.encode(tuple(address _address, uint256[4] blsKey)[]))

Also format of the proposal hash being signed by the validators has changed. Prior to this PR, it was corresponding to BlockHash. However now it is the same as checkpointHash.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

[codecov]

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[goran-ethernal]

LGTM

[b00ris]

LGTM