feat: add fmt nix package

```shell
$ nix run .#fmt
```

does all the automatic formatting the CI checks for.

Signed-off-by: Harald Hoyer <harald@matterlabs.dev>

packages/fmt/default.nix

@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: Apache-2.0
+# Copyright (c) 2024 Matter Labs
+{ lib
+, pkgs
+, teepot
+, ...
+}:
+pkgs.writeShellApplication {
+  name = "fmt-teepot";
+
+  runtimeInputs = with pkgs;
+    [ nixpkgs-fmt coreutils taplo ]
+    ++ teepot.teepot.nativeBuildInputs;
+
+  text = ''
+    # .nix
+    echo "* Formatting nix files"
+    nixpkgs-fmt .
+
+    # .toml
+    echo "* Formatting toml files"
+    taplo fmt
+
+    # .rs
+    echo "* Formatting rust files"
+    cargo fmt
+  '';
+}