Create a BackendConfig for the Ingress (L7 loadbalancer)

A backend config allows us to configure the load balancer.
Here's an example:
https://github.com/sroettger/play-my-challenge/blob/main/pwngdb/backend-config.yaml

Some nice to have config options:
* default timeout > 30s. I needed this in that particular challenge for a long running websocket
* automatic redirect to https

We should expose these things in the challenge.yaml spec