maintainers: require GitHub handle

At the scale of Nixpkgs, actively maintaining a package is only possible
with integration into CI. To be able to be pinged for review requests,
the maintainer must have a GitHub handle, which:
- Leads to an invitation to the NixOS org, which comes with additional
privileges.
- Allows to request the maintainer for review as a member of this org.
- Automatically requests the maintainer for review in CI.

Currently, the GitHub handle is not strictly enforced. This leads to
some new maintainers accidentally forgetting to set these. We can avoid
these mistakes and enforce them via CI.

(cherry picked from commit 568b19f)

Author: wolfgangwalther

lib/tests/maintainer-module.nix

@@ -7,6 +7,12 @@ in
     name = lib.mkOption {
       type = types.str;
     };
+    github = lib.mkOption {
+      type = types.str;
+    };
+    githubId = lib.mkOption {
+      type = types.ints.unsigned;
+    };
     email = lib.mkOption {
       type = types.nullOr types.str;
       default = null;
@@ -15,14 +21,6 @@ in
       type = types.nullOr types.str;
       default = null;
     };
-    github = lib.mkOption {
-      type = types.nullOr types.str;
-      default = null;
-    };
-    githubId = lib.mkOption {
-      type = types.nullOr types.ints.unsigned;
-      default = null;
-    };
     keys = lib.mkOption {
       type = types.listOf (
         types.submodule {