Setup an Hydra declarative project

This commit adds configuration files ingested an Hydra instance to
create a declarative project.
Currently, this Hydra instance builds
- the master branch
- all pull requests

We use the nix community provided Hydra instance
https://hydra.nix-community.org/project/simple-nixos-mailserver.

.hydra/spec.json

@@ -0,0 +1,30 @@
+{
+  "enabled": 1,
+  "hidden": false,
+  "description": "Simple NixOS Mailserver",
+  "nixexprinput": "nixexpr",
+  "nixexprpath": ".hydra/declarative-jobsets.nix",
+  "checkinterval": 60,
+  "schedulingshares": 100,
+  "enableemail": false,
+  "emailoverride": "",
+  "keepnr": 3,
+  "type": 0,
+  "inputs": {
+    "nixexpr": {
+      "value": "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver master",
+      "type": "git",
+      "emailresponsible": false
+    },
+    "nixpkgs": {
+      "value": "https://github.com/NixOS/nixpkgs 0f920b05cbcdb8c0f3c5c4a8ea29f1f0065c7033 ",
+      "type": "git",
+      "emailresponsible": false
+    },
+    "pulls": {
+      "type": "gitlabpulls",
+      "value": "https://gitlab.com 7219050",
+      "emailresponsible": false
+    }
+  }
+}