[BACKEND] Add optional ethnicity field to sign up form

[bcho892]

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

As requested by the university, the club needs to be able to fill in an ethnicity field if desired

The UserAdditionalInfo model and sign up form need to be updated.

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