Allow custom PHP code in the real-time benchmark workflow - #24189

We have a real-time benchmark workflow which can benchmark the difference before and after a PR to measure performance improvements. However, Most of the performance PR focus on a specific function. Those PRs produce little difference in huge frameworks' benchmark (and often arguably within noise) The idea of this PR is to add a feature to the workflow (or expose a already exist feature from kocsismate/php-version-benchmarks@d29cbb9) which allows us to run custom code in real-time benchmark of a PR.

Let's say a PR claims to improve performance of str_contains. Than we can run

<?php
$haystack = str_repeat("abcdef", 1024) . "needle";

for ($i = 0; $i < 100000; $i++) {
    $result = str_contains($haystack, "needle");
}

in that workflow to see real-time benchmark results to check the improvments.

If the custom_code field is empty, than we go back to the original real-time workflow. Only maintainers with push permittions are able to run this workflow.