Create GHA workflow

[mwarkentin]

Add a workflow to ops (or create a reusable workflow in skrooge?) that can be triggered manually and pass in the various options as inputs.

I could see getting a comment-driven workflow going: /skrooge estimate -i c2-standard-30 -r 16 -c 2000 -m 3072 and it responds with an estimate after running the GHA

[mwarkentin]

Super-mega bonus points: figure out how to pull enough information from a PR changing a single k8s value (eg. changing the replica count) in order to get a skrooge estimate

[mwarkentin]

Previous art - install sentry-kube, render.. this could be enough to get the info for running skrooge: https://github.com/getsentry/ops/blob/master/.github/workflows/diff-k8s-render.yaml

[mwarkentin]

Alternatively leverage materialized manifests (snuba-only currently) and use that to push for more manifests across our services.

[mwarkentin]

All services have materialized manifests now.