Update agent session API, to support uploading files while create a new session

[flygithub]

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

• Documentation Update
• Other (please describe):

[KevinHuSh]

Appreciation!
It's really an amazing work if you could update the docs as well.

[flygithub]

• Test Result: