Shared API types between frontend and backend

[mantariksh]

APIs are a contract between the frontend and backend. This means that the client and server should agree on the shape of every API. In order to ensure that the frontend and backend agree on the API shapes without duplicating code, it makes sense for API types to be shared between the frontend and backend.

Implementing this will involve the following steps:

1. Edit the build pipeline to accommodate shared code between the client and server directories.
2. Create base types for each database table and have the model types inherit from the shared base types.
3. Create types for each API's return shape by composing the base types, and use these types in both the frontend services (which call the API) and the backend controllers (which serve the responses).