Replace ctype_digit for PHP 8.1 deprecation of non-string arguments (#1355)

From [php.net's ctype_digit
documentation](https://www.php.net/manual/en/function.ctype-digit.php):

> As of PHP 8.1.0, passing a non-string argument is deprecated. In the
future, the argument will be interpreted as a string instead of an ASCII
codepoint. Depending on the intended behavior, the argument should
either be cast to
[string](https://www.php.net/manual/en/language.types.string.php) or an
explicit call to [chr()](https://www.php.net/manual/en/function.chr.php)
should be made.

## Motivation

Deprecation is throwing a warning when passing a string into ctype_digit
when checking the timestamp.

## Solution

Updated the integer check to move away from `ctype_digit`.

Author: svix-onelson

php/src/Webhook.php

@@ -70,7 +70,7 @@ public function verify($payload, $headers)
 
     public function sign($msgId, $timestamp, $payload)
     {
-        $is_positive_integer = ctype_digit($timestamp);
+        $is_positive_integer = is_numeric($timestamp) && (int) $timestamp == $timestamp && (int) $timestamp > 0;
         if (!$is_positive_integer) {
             throw new Exception\WebhookSigningException("Invalid timestamp");
         }