[BACKEND] Create endpoint for user to sign up to event

[bcho892]

Is your feature request related to a problem? Please describe.

This endpoint should take in the fields specified by #658, and return an appropriate response indicating the success

BEFORE MERGING

• [ ] Storybooks created where possible
• [ ] Tested with nock if data fetching
• [ ] Tests written for critical interactions
• [ ] PR Reviewed (For non-trivial changes)
• [ ] Changes tested after rebasing on master or merging in master (hint: git fetch origin master:master, then git rebase master or git merge master)
• [ ] All required PR checks passing