[BACKEND] Setup endpoint to sign up a user

[choden-dev]

Is your feature request related to a problem? Please describe.
We need a route /signup so that users get an account after providing the required details
blocked by #137
blocked by #192

Describe the solution you'd like
Make use of the UserService and AuthService to create an document in the users collection as well as using the auth service to give them an account. We should return a JWT token for the newly created user that can be used with signInWithCustomToken on the front end

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