[BACKEND] define data structure to manage event sign ups

[choden-dev]

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

IS SEPARATE from #591. This one should include:

• first_name
• last_name
• email
• is_member

Make sure it is documented in the wiki

BEFORE MERGING

• Integration test written for services
• Schemas annotated if adding new models
• Code generation run (hint: yarn workspace server tsoa spec-and-routes)
• Appropriate mocks created where possible
• 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