Utility to validate webhook signatures

I'm not sure this should actually be implemented in this repo, as it is different from other use cases, but maintainers might still decide it fits this repository.

When implementing github webhooks, it is possible to secure them (to reject any fake requests sent to the webhook from elsewhere than Github).

Github sends a signature header in each webhook request. Their documentation shows the Ruby code to validate the request: https://developer.github.com/webhooks/securing/

I suggest providing a utility method which would validate a signature, to avoid having each developer figuring out the corresponding PHP code:

```php
class SignatureChecker
{
function validateSignature(string $signature, string $requestBody, string $secret): bool
{ // TODO implement that
}
}
```

The naming is not final of course.

I suggest that this API takes the 3 strings as arguments rather than alternatives. Here are the ones I considered but which I consider inferior:

- the secret could be a constructor argument, but this could make things harder if you manage multiple webhooks in your codebase and each of them uses a separate secret (you would have to manage separate instances)
- the signature and body could be replaced by a PSR-7 request, from which they would be extracted. But that would make things harder to use for people using frameworks not relying on PSR-7 (Symfony for instance). Having to convert the request to PSR-7 just to check the signature is adding unnecessary overhead (getting the request body and a header are easy enough to be left to the calling code)

The issue provides a proposed SignatureChecker::validateSignature(string $signature, string $requestBody, string $secret) entry point but names no target file or test. First review whether webhook signature validation belongs in this repository and how its existing utilities are organized. Done means the maintainers agree on the API and the utility is implemented with coverage for the documented signature-validation behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.