[RPC StateReader] Add a test helper method for getting block info

[juanbono]

[matias-gonz]

Write a helper function that wraps a RPC call for starknet_getBlockWithTxHashes

The function signature should look like this:

#[cfg(test)]
impl RPCStateReader{
    pub fn get_block_info(&self, starknet_os_config: StarknetOsConfig) -> BlockInfo {}
}

Taken from RPCStateReader tests:

        let rpc_state = RpcState::new(RpcChain::MainNet, BlockValue::Number(90003.into()));
        let get_block_info_params = ureq::json!({
            "jsonrpc": "2.0",
            "method": "starknet_getBlockWithTxHashes",
            "params": [rpc_state.block.to_value()],
            "id": 1
        });
        let block_info: serde_json::Value = rpc_state.rpc_call(&get_block_info_params).unwrap();
        let block_info = BlockInfo {
            block_number: block_info["result"]["block_number"]
                .to_string()
                .parse::<u64>()
                .unwrap(),
            block_timestamp: block_info["result"]["timestamp"]
                .to_string()
                .parse::<u64>()
                .unwrap(),
            gas_price: gas_price,
            sequencer_address: fee_token_address,
        };

Reuse the function in tests when posible