Added create user endpoints

[kilehsu]

Tracking Info

Resolves #9

Changes

Added create user backend that works in both the MongoDB and Firebase. It checks the body to make sure the request is correct and it handles email duplication.

• TODO
  Correctly implementing with firebase
  I used my own firebase config and it seems to work. Not sure if this is how we want to intialize our firebase in the future.

Testing

I have tested to make sure it creates a user

Tested with validator errors

[mraysu]

Looks good, a few things before I can approve.

1. In backend/src/app.ts, we do not need to invoke dotenv.config(), this is what backend/src/config.ts is for. Use config.ts for verifying and exporting backend environment variables.
2. In backend/src/controllers/user.ts, we want to make sure our errors are propagating across our middleware functions. Please add a next: NextFunction parameter to the createUser function, and pass caught errors to said function. example
3. @SaazM created a service account key for us to use and added it to our .env variables. Check the drive for the updated .env so we can save the key as a single shared secret.
4. In backend/src/util/firebase.ts, to initialize our app, you shouldn't need to specify a databaseURL since we aren't using a firebase database.

Please merge your code with main and push these changes.