Add legacy open-source caching module as default

Recover the pre-extraction caching code (from 7a4e0968~1) into a
standalone module at sources/legacy-caching/. This module bundles
@actions/cache@4.0.5 with the existing patch and adapts the old
caching API to the current CacheService interface.

The cache-service-loader now uses the legacy module by default and
only loads the proprietary vendored module when the license is
accepted. Both paths are wrapped in a LoggingCacheService that
identifies which module is active in logs and the Job Summary.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sources/legacy-caching/tsconfig.json

@@ -0,0 +1,15 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "Node16",
+    "moduleResolution": "Node16",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "outDir": "./dist",
+    "rootDir": "./src",
+    "declaration": false,
+    "sourceMap": true
+  },
+  "include": ["src/**/*.ts"]
+}