Rest Api to add downloads [rough implementation]

[talha5389]

I needed rest api to add downloads via another application I have running locally, so I managed to put something together for my needs.

If someone is interested, I can create PR for it https://github.com/talha5389/ab-download-manager/tree/feat/rest-api-add-download

P.S. I primarily do web dev and haven't touched java in 4-5 years (even in android dev) let alone kotlin. I didn't even try to understand existing codebase. Just tried to implement functionality following pattern already in place