Fix declare transactions in function execute_tx_with_blockifier

[FrancoGiachetta]

Currently, when executing declare transaction's, we are using the state in which that transaction occurred. In order to execute these kind of transactions we need its contract class, this is how its done right now:

let block_number = context.block_info().block_number;
let network = parse_to_rpc_chain(&context.chain_info().chain_id.to_string());
// we need to retrieve the next block in order to get the contract_class
let contract_class = state
.get_contract_class(&declare_tx.class_hash())
.unwrap();
let class_info = calculate_class_info_for_testing(contract_class);

The problem here is that get_contract_class(...) will fail since the contract_class has not been delcared yet and therefore it does not exist. One solution would be to create a new reader which is one state upon the transaction's one. However, since this function is used to benchmark purposes, this call would not be cached thus producing a regression.