Create an openapi dev shell

Move the openapi flake to the root level and refactor to a devshell
called `openapi`.

The other flake.nix files will be similarly combined in future commits.

Signed-off-by: Oliver Davies <oliver@oliverdavies.uk>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1758277210,
+        "narHash": "sha256-iCGWf/LTy+aY0zFu8q12lK8KuZp7yvdhStehhyX1v8w=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "8eaee110344796db060382e15d3af0a9fc396e0e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}