Add RPC endpoint that returns the address of the native token

[jrchatruc]

🌟 Feature Request

When the stack is used with an ERC20 as its native token, clients like the explorer need to know which token it is to display information about it (like its name, symbol, decimals, maybe an image, etc).

For this, it's enough that the server has an RPC endpoint zks_getNativeTokenAddress that returns the address of the contract on the L1. With that, clients can call the contract's methods to fetch the information they need.

When the native token is eth, the returned address should be 0.