feat(gcs): Add support for Google Cloud Storage (GCS) integration (#11718)

### 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)

common/settings.py

@@ -31,6 +31,7 @@ import rag.utils.ob_conn
 import rag.utils.opensearch_conn
 from rag.utils.azure_sas_conn import RAGFlowAzureSasBlob
 from rag.utils.azure_spn_conn import RAGFlowAzureSpnBlob
+from rag.utils.gcs_conn import RAGFlowGCS
 from rag.utils.minio_conn import RAGFlowMinio
 from rag.utils.opendal_conn import OpenDALStorage
 from rag.utils.s3_conn import RAGFlowS3
@@ -109,6 +110,7 @@ MINIO = {}
 OB = {}
 OSS = {}
 OS = {}
+GCS = {}
 
 DOC_MAXIMUM_SIZE: int = 128 * 1024 * 1024
 DOC_BULK_SIZE: int = 4
@@ -151,7 +153,8 @@ class StorageFactory:
         Storage.AZURE_SAS: RAGFlowAzureSasBlob,
         Storage.AWS_S3: RAGFlowS3,
         Storage.OSS: RAGFlowOSS,
-        Storage.OPENDAL: OpenDALStorage
+        Storage.OPENDAL: OpenDALStorage,
+        Storage.GCS: RAGFlowGCS,
     }
 
     @classmethod
@@ -250,7 +253,7 @@ def init_settings():
     else:
         raise Exception(f"Not supported doc engine: {DOC_ENGINE}")
 
-    global AZURE, S3, MINIO, OSS
+    global AZURE, S3, MINIO, OSS, GCS
     if STORAGE_IMPL_TYPE in ['AZURE_SPN', 'AZURE_SAS']:
         AZURE = get_base_config("azure", {})
     elif STORAGE_IMPL_TYPE == 'AWS_S3':
@@ -259,6 +262,8 @@ def init_settings():
         MINIO = decrypt_database_config(name="minio")
     elif STORAGE_IMPL_TYPE == 'OSS':
         OSS = get_base_config("oss", {})
+    elif STORAGE_IMPL_TYPE == 'GCS':
+        GCS = get_base_config("gcs", {})
 
     global STORAGE_IMPL
     STORAGE_IMPL = StorageFactory.create(Storage[STORAGE_IMPL_TYPE])