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>

openapi-mock.nix

@@ -0,0 +1,15 @@
+{ buildGoModule, fetchFromGitHub }:
+
+buildGoModule rec {
+  pname = "openapi-mock";
+  version = "0.3.9";
+
+  src = fetchFromGitHub {
+    owner = "muonsoft";
+    repo = pname;
+    rev = "v${version}";
+    hash = "sha256-7u//uwcVV1/EI6Rr3ju7KOwMYt/dXivyvBWIpTaoWZk=";
+  };
+
+  vendorHash = "sha256-KPCRunuCIbBX+YpHgshixmrxM3Ey0LIdEC0Z4CtpQoI=";
+}