feat: add Nix flake

[a-h]

This PR adds a Nix flake to the repo.

If you checkout the code and run nix develop inside the directory, you'll get a shell containing all of the dependencies required to work on deepface.

Running nix run inside the directory results in running the API which can be tested with a curl command, e.g.:

curl -H "Content-Type: application/json" \
  --data '{ "img_path": "/home/adrian/face-images/couple.jpg" }' \
  http://localhost:5000/represent

[serengil]

Prefer to get opencv path dynamically instead of env vars. Would you please revert this?

[serengil]

shall we move this to a folder instead of a root? otherwise it will make project messy.