API: Download doc api (#1354)

### What problem does this PR solve?

Adds download_document api

### Type of change


- [x] New Feature (non-breaking change which adds functionality)

sdk/python/ragflow/ragflow.py

@@ -12,12 +12,12 @@
 #  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
-
 import json
 import os
 
 import requests
 
+from api.db.services.document_service import DocumentService
 from api.settings import RetCode
 
 
@@ -126,7 +126,22 @@ class RAGFlow:
         return response.json()
 
     # ----------------------------download a file-----------------------------------------------------
+    def download_file(self, dataset_id, document_id):
+        endpoint = f"{self.dataset_url}/{dataset_id}/documents/{document_id}"
+        res = requests.get(endpoint, headers=self.authorization_header)
 
+        content = res.content  # binary data
+        # decode the binary data
+        try:
+            decoded_content = content.decode("utf-8")
+            json_data = json.loads(decoded_content)
+            return json_data  # message
+        except json.JSONDecodeError:  # binary data
+            _, document = DocumentService.get_by_id(document_id)
+            file_path = os.path.join(os.getcwd(), document.name)
+            with open(file_path, "wb") as file:
+                file.write(content)
+            return {"code": RetCode.SUCCESS, "data": content}
     # ----------------------------start parsing-----------------------------------------------------
 
     # ----------------------------stop parsing-----------------------------------------------------
@@ -144,3 +159,4 @@ class RAGFlow:
     # ----------------------------get a specific chunk-----------------------------------------------------
 
     # ----------------------------retrieval test-----------------------------------------------------
+