Make the validator pay the penalty

[aakoshh]

Related to #349

A miner is allowed to include a transaction where the gas_fee_cap is lower than the prevailing base_fee, which results in a penalty being calcualted, which the miner should pay. This mechanism is not implemented, however, so any validator can include any cheapo transactions that everyone has to validate.

See what Lotus / Forest do in this regard and implement a similar mechanism.

Questions:

• What if the miner runs out of funds after transaction execution?
• How should we decrease the miner voting power if they can't pay the penalty?

[raulk]

Have we considered simply disallowing the inclusion of such messages?

[aakoshh]

@raulk with vanilla ABCI disallowing the inclusion of such messages would not have been possible because the first time Fendermint would see the block would be when they are already finalized, however with ABCI++ we can make this a criteria for voting in process_proposal, so we have this option too.

So we can either 1) not vote on a block we don't like or 2) allow the block but penalize the validator by making them pay and/or reducing their voting power.

[raulk]

@aakoshh (1) seems simplest and viable to implement at this stage. (2) is way more complex as you need to guarantee that there's a slashable amount either by having locked rewards, block minting rewards, or by slashing the stake/weight at the parent which involves a bottom-up interaction. Feel free to open an issue to consider (2) for the future; right now with federated validation it doesn't make sense.

[raulk]

@aakoshh why wouldn't we be able to reject the transaction in check_tx? Does CometBFT only invoke this on mempool gossip and not when validating a block? (That would be assymetrical and inconsistent?)