feat(rpc): add getHeader by number+hash

[alrevuelta]

Add support for eth_getHeaderByNumber and eth_getHeaderByHash endpoints. To be used as follows:

use ethers_core::types::{BlockId, BlockNumber};
use ethers_providers::Middleware;
use ethers_providers::{Http, Provider};

async fn mytest() {
    let provider = Provider::<Http>::try_from("http://localhost:8545").unwrap();
    let by_num = BlockId::Number(BlockNumber::Number(18826806u64.into()));
    let by_hash = BlockId::Hash(
        "0xac45643453aba125a85a779163e7cfa40e4f8898639f1d5249de1c1481817510".parse().unwrap(),
    );
    // Use by_num or by_hash
    let block = provider.get_header(by_hash).await.unwrap();
    println!("block: {:?}", block);
}

Note that the same type Block is used, but non-header fields are left empty.

[mattsse]

while this is not quite right, because this returns a header, this will still succeed because the transactions will be empty.

since we don't have a header type I'm okay with this