### 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):
### What problem does this PR solve?
Added static check at PR CI
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Refactoring
### What problem does this PR solve?
Add api for list agents and agent seesions
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>
