Feat: Add http api to create, update, or delete agents. (#7515)

### What problem does this PR solve?

Hello, we are using ragflow as a backend service, so we need to manage
agents from our own frontend. So adding these http APIs to manage
agents.

The code logic is copied and modified from the `rm` and `save` methods
in `api/apps/canvas_app.py`.

btw, I found that the `save` method in `canvas_app.py` actually allows
to modify an agent to an existing title, so I kept the behavior in the
http api. I'm not sure if this is intentional.

### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

sdk/python/ragflow_sdk/ragflow.py

@@ -244,3 +244,49 @@ class RAGFlow:
                 result_list.append(Agent(self, data))
             return result_list
         raise Exception(res["message"])
+
+    def create_agent(self, title: str, dsl: dict, description: str | None = None) -> None:
+        req = {
+            "title": title,
+            "dsl": dsl
+        }
+
+        if description is not None:
+            req["description"] = description
+
+        res = self.post("/agents", req)
+        res = res.json()
+
+        if res.get("code") != 0:
+            raise Exception(res["message"])
+
+    def update_agent(
+        self,
+        agent_id: str,
+        title: str | None = None,
+        description: str | None = None,
+        dsl: dict | None = None
+    ) -> None:
+        req = {}
+
+        if title is not None:
+            req["title"] = title
+
+        if description is not None:
+            req["description"] = description
+
+        if dsl is not None:
+            req["dsl"] = dsl
+
+        res = self.put(f"/agents/{agent_id}", req)
+        res = res.json()
+
+        if res.get("code") != 0:
+            raise Exception(res["message"])
+
+    def delete_agent(self, agent_id: str) -> None:
+        res = self.delete(f"/agents/{agent_id}", {})
+        res = res.json()
+
+        if res.get("code") != 0:
+            raise Exception(res["message"])