CRAVEX-integration: Packaging for Nix

[pombredanne]

Nix packaging scripts/derivations

[chinyeungli]

Note that the default.nix need to be update manually whenever the packages version or dependencies change in pyproject.toml.

@pombredanne Should I create a script to auto generated the default.nix instead?

[chinyeungli]

as our doc, https://dejacode.readthedocs.io/en/latest/installation.html , states the easiest way is to run with Docker. I am not sure the purpose of this ticket.
Do we want to have another installation method such that once it's isntalled, One can do dejacode to run the application instead of docker compose up?