[BACKEND] Create endpoint for admins to add coupons to user

[bcho892]

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

Note that these are essentially $40 off coupons, we need to know the user's stripe id from the firebase document (endpoint should take in a user's uid, and also quantity of coupons to be added). BEFORE MERGING

• [x] Integration test written for services
• [x] Schemas annotated if adding new models
• [x] Code generation run (hint: yarn workspace server tsoa spec-and-routes)
• [x] Appropriate mocks created where possible
• [ ] PR Reviewed (For non-trivial changes)
• [x] Changes tested after rebasing on master or merging in master (hint: git fetch origin master:master, then git rebase master or git merge master)
• [x] All required PR checks passing