[BACKEND] Add a signup timestamp to the EventReservation Type

[bcho892]

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

Firebase is unhelpful and doesn't automatically provide a creation timestamp for documents. This Ticket requires us to extend the EventReservation interface to allow us to track when the signup was made.

This timestamp should be omitted from the /events/signup body.

For an example of how this can be implemented see this example from server/src/service-layer/controllers/AdminController.ts

 /**
       * Log that there was a positive change in availability for a date range
       */
      await new BookingHistoryService().addAvailibilityChangeEvent({
        timestamp: Timestamp.now(),
        start_date: startDate,
        end_date: endDate,
        event_type: "changed_date_availability",
        change: slots || DEFAULT_BOOKING_MAX_SLOTS
      })

where Timestamp.now() is used.

BEFORE MERGING

• [ ] Integration test written for services
• [ ] Schemas annotated if adding new models
• [x] 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