[BACKEND] Create an EventService for CRUD on Events

[choden-dev]

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

Refer to the data structures defined in #658 #591, considering the following functionalities, alongside basic CRUD:

• Adding a user to an event
• getting all signups for an event
• removing user from event

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