build-aux: Add versions to Flatpak plugin extension point

- Enables access to stable branch (Flathub) plugin to Flathub Beta releases
- Strip away access to beta branch plugins (Flathub Beta), mostly unused
  by submitters
- Enables test Flatpak plugin from Flathub submissions to be tested

build-aux/com.obsproject.Studio.json

@@ -22,6 +22,7 @@
     "add-extensions": {
         "com.obsproject.Studio.Plugin": {
             "directory": "plugins",
+            "versions": "stable;test",
             "subdirectories": true,
             "add-ld-path": "lib",
             "merge-dirs": "lib/obs-plugins;share/obs/obs-plugins",