[php-src] PR #24189: Allow custom PHP code in the real-time benchmark workflow

Pull Request: https://github.com/php/php-src/pull/24189 Author: LamentXU123 We have a real-time benchmark workflow which can benchmark the different 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 workflow (or expose a already exist feature from https://github.com/kocsismate/php-version-benchmarks/commit/d29cbb9954037c3368e02fd9981b85c3bb77fe78) 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.