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

[choden-dev]

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