[RFC-191]: EVM-740 - Move distribute rewards to first block in epoch

[goran-ethernal]

Description

ValidatorSet contract on supernet is used as a wrapper around staked tokens by validators on L2. For governance, it is used as a voting token as well, which makes sense, since we are only allowing validators to propose and vote for governance proposals.

But, to be able to use ValidatorSet as vote token in governance, we needed to change it to implement ERC20VotesUpgradable implementation, which changed how the contract checkpoints its total supply at a certain point in time, and how it returns totalSupplyAt, which is used by RewardPool contract to accurately distribute rewards.
Now totalSupplyAt has a check which prevents calling it for the last checkpointed total supply, if they happen in the same block, and because of that, reward distribution failed.

This PR moves reward distribution to the first block of epoch. So when an epoch completes, and gets committed (which will checkpoint total supply at ValidatorSet contract), the new epoch will start, and in the first block of new epoch, proposer will add the distribute rewards transaction for the previously completed epoch.

This has three benefits:

1. It lowers gas consumption on epoch ending blocks.
2. It enables more accurate reward distribution.
3. We can use ValidatorSet as voting token for governance.

IMPORTANT NOTE:
Rewards will only be distributed in the first block of an epoch if governance fork is enabled. If it is not enabled, rewards will be distributed old way, at epoch ending block.

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

[Stefan-Ethernal]

Overall looks good!