[BACKEND] Create endpoint for creating guest user

[bcho892]

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

The request body should have the UserAdditionalInfo keys with the stripe_id omitted. The endpoint should be created in server\src\service-layer\controllers\AdminController.ts.

You will have to make use of the UserDataService to create a new document.

Following this, you should return the uid of the user that was created. Note that the ResponseModels must extend CommonResponse so we have a defined data format in case of an error.

Tests MUST be written for this in routes.test.ts - use the command yarn firebase-test "jest routes" to test this.

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

[bcho892]

we already have this actually with admin/users/create...