Add shell.nix.

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This PR adds a shell.nix script, which sets up a development environment which lets people using Nix and NixOS build the project.

If you find yourself wondering if writing this kind of scripts is a bit painful, the answer is yes.

I don't really understand what is happening in this script and how to update it if something changes in Qdrant tooling. So I am not sure if we are ready to take this responsibility

Very understandable. I needed to write it to build the project myself, so making a PR for it once it was done seemed logical.