Eliminate TODO: use multicall in extensions

[shuhuiluo]

There are some TODOs left in the extensions mod, pending an async multicall implementation using alloy.

While an explicit multicall approach in ethers-rs works, it's not elegant. An alloy::providers::Provider should batch the calls under the hood without any code change at the call site.

[byhashdong]

It seems promising, yet obtaining support for alloys is not straightforward.