Add GCS cache backend with GitHub fallback

Port GCS support from danySam/gcs-cache. When gcs-bucket input is set,
cache to Google Cloud Storage via Application Default Credentials; else
fall back to GitHub's cache service transparently.

- New inputs: gcs-bucket, gcs-path-prefix (default: github-cache)
- New src/utils/gcsCache.ts wraps @actions/cache restore/save
- restoreImpl/saveImpl swap @actions/cache import for local wrapper
- Adds @google-cloud/storage ^7.16.0, bumps typescript to ^5.8.3
- Regenerated dist/ via npm run build

Skipped: upstream workflow removals, README/examples/RELEASES churn,
licensed/codeql config, GCS integration-test workflow.

action.yml

@@ -1,6 +1,6 @@
-name: 'Cache'
-description: 'Cache artifacts like dependencies and build outputs to improve workflow execution time'
-author: 'GitHub'
+name: 'Cache with GCS Support'
+description: 'Cache artifacts to Google Cloud Storage or GitHub with automatic fallback'
+author: 'danySam'
 inputs:
   path:
     description: 'A list of files, directories, and wildcard patterns to cache and restore'
@@ -26,6 +26,13 @@ inputs:
     description: 'Check if a cache entry exists for the given input(s) (key, restore-keys) without downloading the cache'
     default: 'false'
     required: false
+  gcs-bucket:
+    description: 'Google Cloud Storage bucket name to use for caching. When provided, GCS will be used as the cache backend.'
+    required: false
+  gcs-path-prefix:
+    description: 'Optional prefix path within the GCS bucket for cache files'
+    required: false
+    default: 'github-cache'
   save-always:
     description: 'Run the post step to save the cache even if another step before fails'
     default: 'false'
@@ -38,7 +45,7 @@ outputs:
   cache-hit:
     description: 'A boolean value to indicate an exact match was found for the primary key'
 runs:
-  using: 'node24'
+  using: 'node20'
   main: 'dist/restore/index.js'
   post: 'dist/save/index.js'
   post-if: "success()"