Add Flatpak CI

This is a merge request for the task assigned to me for SoK-23 at KDE. Have added the manifest files for flatpak CI integration in the application.
@ravier @apol

.flatpak-manifest.json

@@ -0,0 +1,57 @@
+{
+    "id": "org.kde.kigo",
+    "runtime": "org.kde.Platform",
+    "runtime-version": "5.15-22.08",
+    "sdk": "org.kde.Sdk",
+    "command": "kigo",
+    "rename-icon": "kigo",
+    "finish-args": [
+        "--share=ipc",
+        "--socket=x11",
+        "--socket=wayland",
+        "--socket=pulseaudio",
+        "--device=dri"
+    ],
+    "modules": [
+        {
+            "name": "libkdegames",
+            "buildsystem": "cmake-ninja",
+            "sources": [
+                {
+                    "type": "git",
+                    "url": "https://invent.kde.org/games/libkdegames",
+                    "branch": "master"
+                }
+            ]
+        },
+        {
+            "name": "gnugo",
+            "build-options": {
+                "cflags": "-fcommon"
+            },
+            "sources": [
+                {
+                    "type": "archive",
+                    "url": "https://ftp.gnu.org/gnu/gnugo/gnugo-3.8.tar.gz",
+                    "sha256": "da68d7a65f44dcf6ce6e4e630b6f6dd9897249d34425920bfdd4e07ff1866a72",
+                    "x-checker-data": {
+                        "type": "anitya",
+                        "project-id": 1209,
+                        "stable-only": true,
+                        "url-template": "https://ftp.gnu.org/gnu/gnugo/gnugo-$version.tar.gz"
+                    }
+                }
+            ]
+        },
+        {
+            "name": "kigo",
+            "buildsystem": "cmake-ninja",
+            "sources": [
+                {
+                    "type": "dir",
+                    "path": "."
+                }
+            ]
+        }
+    ]
+}

.flatpak-manifest.json.license

@@ -0,0 +1,2 @@
+# SPDX-FileCopyrightText: None
+# SPDX-License-Identifier: CC0-1.0

.gitlab-ci.yml

@@ -8,3 +8,4 @@ include:
   - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/linux-qt6.yml
   - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/freebsd-qt6.yml
   - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/windows-qt6.yml
+  - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/flatpak.yml