docs: add comment about potential overflow of base fee per gas

[fgimenez]

Base fee in EIP1559 is allowed to increase unbounded, and this is not reflected when it is converted from a UINT256 to a u64. It is improbable for it to be bigger than u64 max (would mean ~18 ETH per gas), the PR adds a comment about this.

[mattsse]

we recently unified all fee types in alloy to u128:

https://github.com/alloy-rs/alloy/blob/07611cfc9f277dc5bbfd7d21e9d70ad37b441a51/crates/rpc-types/src/eth/block.rs#L112-L122

but not the engine types:

https://github.com/alloy-rs/alloy/blob/07611cfc9f277dc5bbfd7d21e9d70ad37b441a51/crates/rpc-types-engine/src/payload.rs#L157-L165

we should unify the engine types to u128 as well: alloy-rs/alloy#752

[fgimenez]

@mattsse 👍 thx for the heads up! I see that base_fee_per_gas is already a U256 in engine types https://github.com/alloy-rs/alloy/blob/07611cfc9f277dc5bbfd7d21e9d70ad37b441a51/crates/rpc-types-engine/src/payload.rs#L172 but we are casting it to u64 here. Should we instead cast to u128 here too?

[mattsse]

yeah we should unify all of the fee fields to u128

[fgimenez]

I see there's work in progress here alloy-rs/alloy#753 to unify the engine types to u128, will update this one when it is merged.

[mattsse]

lgtm

I decided against unifying the engine types after having a closer look at the specs which require the types to be 64bits