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.

[baitcode]

@ericnordelo do you accept external contributions?

[ericnordelo]

Hey @baitcode, yes, we happily accept external contributions! Always confirm with us before starting to work on an issue, since we may not accept external contributions for specific ones depending on priorities, complexity, maturity, etc... but this one is not the case, feel free to open a PR and we will review it.

[baitcode]

@ericnordelo
This time I was just asking, already have responsibility for 4 issues. But will watch your repos from now on. Thank you!