[BACKEND] Extend BookingService to track user event signups

[choden-dev]

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

1. Create a new collection to store history for booking signups
2. Create a new service to track booking signup history
   a. We need to know the date range they booked
   b. This must be integration tested
3. Call this service method in the stripe webhook server/src/service-layer/controllers/StripeWebhook.ts

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