feat: add GCP Cloud Storage bucket and object operations

[atharva-nagane]

Context

gcp_storage.rs in the storage module actually implements Compute persistent
disk operations (create_disk, delete_disk, create_snapshot) — not Cloud Storage.
GCS bucket and object operations are entirely missing from the project.

This PR adds a dedicated gcp_object_storage.rs implementing the GCS JSON API.

Operations to implement

Function	Description
create_bucket	Create a new GCS bucket in a given location
delete_bucket	Delete an empty bucket
list_buckets	List all buckets in a project
upload_object	Upload bytes to a named object
download_object	Download object content as bytes
delete_object	Delete an object from a bucket
list_objects	List objects with optional prefix filter

Acceptance criteria

• GcsClient struct following existing GCP struct pattern
• All 7 operations using GCS JSON/upload APIs
• Wired in main.rs and tests/mod.rs
• 5 tests + example doc at examples/gcp/storage/cloud_storage.md
• README updated (GCP Storage row)