feat(cache-magento): add stamp caching for vendor/ directory (#245)

Adds an opt-in `stamp` mode that caches the extracted `vendor/`
directory in addition to the Composer download cache. On a warm
hit, `composer install` is effectively a no-op and shaves 2–5
minutes off a full Magento install.

cache-magento/fixtures/composer.lock

@@ -0,0 +1,9 @@
+{
+    "packages": [
+        {
+            "name": "magento/product-community-edition",
+            "version": "2.4.7"
+        }
+    ],
+    "packages-dev": []
+}