created get_dataset, update_dataset API and fixed: delete (#1201)

### What problem does this PR solve?

Added get_dataset and update_dataset API.
Fixed delete_dataset.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

docs/references/ragflow_api.md

@@ -55,7 +55,8 @@ You are *required* to save the `data.id` value returned in the response data, wh
 {
   "code": 0, 
   "data": {
-    "dataset_name": "kb1"
+    "dataset_name": "kb1",
+    "dataset_id": "375e8ada2d3c11ef98f93043d7ee537e"
   }, 
   "message": "success"
 }