Create a re-usable Github Workflow

[srid]

Similar in spirit to DetSys/ci but specifically for om ci. Also support passing the out paths (via JSON; #270) to other steps for further processing (e.g.: pushing the docker image to registry).

End result: projects like haskell-template or nammayatri can use this workflow, to keep their own .yml file smaller and simpler, without repetitive CI steps.

Tasks

• JSON support (om ci JSON output #270, ci: Replace --json with --write-results #273)
• Decouple our monolithic workflow file
  • Leverage --write-results in CI #275
  • ci: Decouple website workflow #276
  • Move static binary stuff to its own workflow
• Create the re-usable workflow
• Test in other projects (services-flake, nammayatri, etc.)