feat: add support for structured logging with the frankenphp_log() PHP function (#1979)

As discussed in https://github.com/php/frankenphp/discussions/1961,
there is no real way to pass a severity/level to any log handler offered
by PHP that would make it to the FrankenPHP layer. This new function
allows applications embedding FrankenPHP to integrate PHP logging into
the application itself, thus offering a more streamlined experience.

---------

Co-authored-by: Quentin Burgess <qutn.burgess@gmail.com>
Co-authored-by: Kévin Dunglas <kevin@dunglas.fr>

testdata/log-frankenphp_log.php

@@ -0,0 +1,19 @@
+<?php
+
+require_once __DIR__.'/_executor.php';
+
+return function () {
+	frankenphp_log("some debug message {$_GET['i']}", FRANKENPHP_LOG_LEVEL_DEBUG, [
+		"key int"    => 1,
+	]);
+
+	frankenphp_log("some info message {$_GET['i']}", FRANKENPHP_LOG_LEVEL_INFO, [
+		"key string" => "string",
+	]);
+
+	frankenphp_log("some warn message {$_GET['i']}", FRANKENPHP_LOG_LEVEL_WARN);
+
+	frankenphp_log("some error message {$_GET['i']}", FRANKENPHP_LOG_LEVEL_ERROR, [
+		"err" => ["a", "v"],
+	]);
+};