Add assert_valid_signature utility

[ericnordelo]

🧐 Motivation

We have a few places where this code snippet is being used:

let is_valid_signature_felt = ISRC6Dispatcher { contract_address: voter }
                .is_valid_signature(hash, signature.into());

// 3. Check either 'VALID' or true for backwards compatibility
let is_valid_signature = is_valid_signature_felt == starknet::VALIDATED
                || is_valid_signature_felt == 1;

assert(is_valid_signature, Errors::INVALID_SIGNATURE);

We can refactor it into a single utility to avoid repeating it further.