[BACKEND] Change membership endpoint to allow client to select membership type

[bcho892]

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

Right now we select the membership the user gets based on their user data (#278), however it was agreed upon with the client that this causes issues for graduates etc who may not be able to provide the relevant details.

We must now let the membership endpoint take in a request body that specifies the membership type given in the format MembershipTypeValues

Most of the logic should be kept the same.

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