…ote storage

Replace cloud_storage crate with object_store crate for GCS remote filesystem.

Problem:
- cloud_storage crate requires SERVICE_ACCOUNT or SERVICE_ACCOUNT_JSON env vars
- Panics without them on GKE with Workload Identity (WIF) configured
- Affects all GKE users running CubeStore with Workload Identity

Fix:
- Switch to object_store crate (already a transitive dep via Arrow/DataFusion)
- GoogleCloudStorageBuilder::from_env() supports full ADC credential chain:
  1. CUBESTORE_GCP_KEY_FILE (backward compat)
  2. CUBESTORE_GCP_CREDENTIALS (backward compat)
  3. GKE Workload Identity via metadata server (the main fix)
  4. GOOGLE_APPLICATION_CREDENTIALS file
  5. gcloud CLI credentials (dev machines)

Backward compatible: existing SA key file deployments continue to work unchanged.

Closes cube-js#9837
Closes cube-js#7279

…nt, comments

- Upload: replace fs::read() full-buffer with streaming PutPayload::from_stream()
- Download: replace bytes().await full-buffer with chunked stream to disk
- Add legacy mapping for SERVICE_ACCOUNT_JSON, GOOGLE_APPLICATION_CREDENTIALS_JSON
- Fix page count ceiling division: (len + 999) / 1000
- Fix head() comment: workaround is for object_store path handling, not GCS consistency

…tResult

- Remove leftover second store.put(data) call in upload_file (data undefined)
- Remove get_result.bytes().await before into_stream() — both consume GetResult
- Fix misleading 'GCS consistency error' wording in check_upload_file