### What problem does this PR solve?
Update the agent session API "POST /api/v1/agents/{agent_id}/sessions",
to support uploading files while create a new session:
- currently, the API only supports requesting with a json body. If user
wants to upload a doc or image when create session, like what is already
supported on the web client, we need to update the API.
- if upload an image, ragflow will call image2text, and a user_id is
needed for the image2text model. So we need to send user_id in the API
request. As form-data is needed to upload files, not json body, seems we
need to put the user_id in the url as an optional parameter (currently
user_id is an optional in json body).
### Type of change
- [x] Documentation Update
- [x] Other (please describe):
409310aae9