feature #50391 [FrameworkBundle][HttpKernel] Introduce $buildDir argument to WarmableInterface::warmup to warm read-only artefacts in build_dir (Okhoshi)

This PR was squashed before being merged into the 6.4 branch.

Discussion
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[FrameworkBundle][HttpKernel] Introduce `$buildDir` argument to `WarmableInterface::warmup` to warm read-only artefacts in `build_dir`

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #50357
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

See #50357 for the details and the reproduction steps. In this PR, I also moved `ConfigBuilderCacheWarmer` to use the `buildDir` argument.

Commits
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8b604ff3ae [FrameworkBundle][HttpKernel] Introduce `$buildDir` argument to `WarmableInterface::warmup` to warm read-only artefacts in `build_dir`

Author: nicolas-grekas

CacheWarmer/ProxyCacheWarmer.php

@@ -38,9 +38,9 @@ public function isOptional(): bool
     }
 
     /**
-     * @return string[] A list of files to preload on PHP 7.4+
+     * @param string|null $buildDir
      */
-    public function warmUp(string $cacheDir): array
+    public function warmUp(string $cacheDir /* , string $buildDir = null */): array
     {
         $files = [];
         foreach ($this->registry->getManagers() as $em) {