[BACKEND] Send confirmation when admin adds a user using the add booking endpoint

[bcho892]

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

It would be helpful that the admins don't need to manually follow up - changes should utilised/create new methods in the MailService

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