Add eth_multicall

[zemse]

The eth_call executes every message call on the latest state and then discards the state. When someone wants to inspect a state that would be after a transaction would go through, currently it involves a non cheap way, i.e. using the mainnet fork (like ganache/hardhat network/anvil) to confirm the first tx and then query state on it, which makes a lot of rpc calls. This PR proposes addition of eth_multicall to the JSON-RPC spec.

eth_multicall([]transction) -> []bytes

This method enables following use cases:

• easy state inspection after a series of transactions.
• batching of multiple eth_calls from frontends, similar to makerdao/multicall (more efficiently since it does not need to execute multicall contract code)

Closes #267

[lightclient]

I think this proposal is a good idea. I recommend as a next step requesting feedback on ACD. The next one is tomorrow!

[lightclient]

@s1na is currently working on a very similar endpoint here: ethereum/go-ethereum#25743
@zemse would be great if you could take a look!

[zemse]

Sure thanks!

[zemse]

Closing in favor of PR #312. The return interface specified over there is array of CallResult which I think it is superior than having an array of bytes since it has the potential to include extra execution details like gasUsed if needed in the future, in a non-breaking way.