feat: Emit Aurora event in ETH exit precompile.

[paouvrard]

In order to improve the bridge UI experience, we need to query for all the transfers initiated by an account on Aurora.
When withdrawing a bridged ERC-20, withdrawToEthereum or withdrawToNear are called and emit a Transfer event when _burn is executed:

aurora-engine/etc/eth-contracts/contracts/EvmErc20.sol

Line 64 in bb21459

_burn(_msgSender(), amount);

We can then query and filter all those events to find the transfers.

However when withdrawing ETH, no event is emitted. Would it be possible to also emit an exit event for ETH from the precompile ?

[paouvrard]

Above I mentioned that ERC-20 exits could be found by querying burn events: Transfer(account, address(0), amount).

Since the same burn event is emitted for withdrawToEthereum and withdrawToNear, we can check if the Aurora tx data contains the sigHash of one of those those 2 functions to differentiate the type of exit. Note that this method would not work for specific multisig transactions which record approvals on-chain (the tx data would not contain the sigHash in that case).

This is quite a rare scenario as multisig wallets like Argent gather signatures off-chain and I don't think these types of legacy gnosis on-chain signatures are common (except in DAOs). So we may consider emitting a custom event for ERC-20 exits also, but it doesn't seem necessary at this time.

[artob]

@sept-en Please work this into your near-term roadmap and communicate a schedule to the Product team.

[sept-en]

@artob Ok, sure.

[birchmd]

Fixed in #288 + #292