Add shell.nix

With this on nixos the server can be launched with:

```sh
nix-shell --command ./otsd
```

Approach suggested https://wiki.nixos.org/wiki/Python

In comparison to the requirements.txt we are not specifying exact
version for each lib, in case of incompatibility deps must be packaged
like suggested in the mentioned guide.

shell.nix

@@ -0,0 +1,18 @@
+# shell.nix
+let
+  # We pin to a specific nixpkgs commit for reproducibility.
+  # Last updated: 2024-04-29. Check for new commits at https://status.nixos.org.
+  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/cf8cc1201be8bc71b7cbbbdaf349b22f4f99c7ae.tar.gz") {};
+in pkgs.mkShell {
+  packages = [
+    (pkgs.python3.withPackages (python-pkgs: [
+      python-pkgs.opentimestamps
+      python-pkgs.leveldb
+      python-pkgs.pystache
+      python-pkgs.requests
+      python-pkgs.qrcode
+      python-pkgs.simplejson
+      python-pkgs.bitcoinlib
+    ]))
+  ];
+}