builder-manifest: Allow all custom appstream tags in catalogue

This allows all `<custom>` tags from metainfo files to reach
catalogue data. Custom tags are used by Flathub to store verification
and build information and often downstream vendors who use Flathub's
catalogue for their own app store want custom tags to identify or
highlight their own apps. For example cosmic using Flathub catalogue
data to highlight cosmic apps on cosmic store app etc.

Allowing individual custom tags one by one is annoying and this is what
libappstream-glib did before the port in 1.3.4. So this brings back
the old as-glib behaviour that was lost in >=1.3.4

Author: bbhtt

src/builder-manifest.c

@@ -2447,6 +2447,8 @@ builder_appstreamcli_compose (const gchar *origin,
   asc_compose_set_media_result_dir (compose, media_dir);
   asc_compose_set_hints_result_dir (compose, hint_dir);
 
+  asc_compose_add_flags (compose, ASC_COMPOSE_FLAG_PROPAGATE_CUSTOM);
+
   g_autoptr(GPtrArray) results = asc_compose_run (compose, NULL, error);
   if (results == NULL)
     {