Backend: optional bonus challenge

In addition to the Google proxy, I’d like to add a second part to the challenge, not directly related to Sunrise.

I’m not sure it should be available directly in the jobs repo, that's why I marked it as optional, but in any case I’d like to use it as a follow up on applicants who pass the Google Proxy test.

- If an applicant completes it from the start although it is marked as optional (if we decide to make it public), it shows motivation imo
- It requires organization into smaller functions, otherwise it's almost an instant fail
- While the Google proxy displays skills in matter of API/requests/server, this challenge is more about abstraction and algorithm
- It's more complicated than it looks :)