Improve Nix support

[srid]

This is follow up to #9

• [x] Nixify shared-kernel and beckn-gateway, and decouple much of the overrides to them
• [ ] Autoformatting and linting
  • [x] Put nix/treefmt.nix module in a shared place for sharing with these repos
  • [x] Enable all flake checks
  • [ ] Enable hlint: https://github.com/nammayatri/nammayatri/issues/31
  • [x] https://github.com/numtide/treefmt-nix/issues/49
  • [x] https://github.com/srid/haskell-flake/issues/97
  • [x] Optional https://github.com/srid/haskell-flake/issues/76
• [x] Avoid filtering self'.packages using https://github.com/srid/haskell-flake/issues/74#issuecomment-1424309168
• [ ] Add .vscode directory settings

[srid]

This is mostly done, but moving pending nixification (for dev environment) to: https://github.com/nammayatri/nammayatri/issues/356