### What problem does this PR solve?
This Pull Request introduces native support for Google Cloud Storage
(GCS) as an optional object storage backend.
Currently, RAGFlow relies on a limited set of storage options. This
feature addresses the need for seamless integration with GCP
environments, allowing users to leverage a fully managed, highly
durable, and scalable storage service (GCS) instead of needing to deploy
and maintain third-party object storage solutions. This simplifies
deployment, especially for users running on GCP infrastructure like GKE
or Cloud Run.
The implementation uses a single GCS bucket defined via configuration,
mapping RAGFlow's internal logical storage units (or "buckets") to
folder prefixes within that GCS container to maintain data separation.
This architectural choice avoids the operational complexities associated
with dynamically creating and managing unique GCS buckets for every
logical unit.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
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