Replace knative with keda

[wilsonianb]

Due to #10 and the fact that knative is has a large footprint and supports may functionality than we need (revisions), try to replace with keda + prometheus (for scaling based on http requests)
https://keda.sh/scalers/prometheus/

This could let us use the ingress controller of our choosing (including k3s's default traefik).

It may require excessive polling, both by prometheus and keda to achieve minimal cold starts.

osiris is mentioned a solution for http request events with keda, but it seems very WIP.