Feat: Add attendance into return body of /attend-event

[ethxng]

In the /attend-event endpoint, we should return the number of times a hacker has attended that event before, specifically the return body of statusCode 409 and 200. Add in the attribute attendance: number in the return body.

• Adjust unit test if necessary.
• I've updated the doc so no need to update it yourself.
• Deadline: Tuesday, Oct 13