feat: implement eth_coinbase/anvil_setCoinbase API endpoints

[itegulov]

Two methods to get/set coinbase (aka author/beneficiary/miner):

#[rpc(name = "anvil_setCoinbase")]
fn set_coinbase(&self, address: Address) -> RpcResult<()>;

#[rpc(name = "eth_coinbase")]
fn coinbase(&self) -> RpcResult<Address>;

I feel like this is of very limited usefulness due to the following reasons:

1. The handler for eth_coinbase is usually disabled for base ETH. I tried 15 different providers and only one of them returned 0x00..00, all others responded with either "unimplemented" or "etherbase must be explicitly specified" (Geth default when --etherbase param is missing).
2. There is a good reason for the above: coinbase is a consensus layer thing now and it does not make sense for it to be present on the execution layer.
3. Neither reth nor anvil support eth_coinbase (weirdly enough latter supports anvil_setCoinbase, maybe to manually replicate EVM executions that depend on COINBASE opcode?)

At the same time cost of "faking" support for it is negligible.

[itegulov]

@dutterbutter @popzxc What do you guys think would make the most amount of sense here? I am leaning toward adding a very simple implementation (Arc<RwLock<Address>> that is only used in these two methods) and printing something in the logs about coinbase having no impact on anything in ZKsync.

Also fine with marking them as intentionally unsupported with a similar message.

[popzxc]

Let's mark them unsupported and wait until someone needs them. Otherwise we're risking adding unneeded functionality.