app-filter: Add serialize and deserialize methods

Add methods to serialise and deserialise the app filter, and use them to
replace the code in `MctManager` which was previously doing this. This
exposes the variant format for the app filter in the API (although the
format is described as ‘opaque’) so that user code can log it or store
it separately.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

libmalcontent/app-filter.h

@@ -96,6 +96,11 @@ MctAppFilterOarsValue   mct_app_filter_get_oars_value    (MctAppFilter *filter,
 gboolean                mct_app_filter_is_user_installation_allowed   (MctAppFilter *filter);
 gboolean                mct_app_filter_is_system_installation_allowed (MctAppFilter *filter);
 
+GVariant     *mct_app_filter_serialize   (MctAppFilter  *filter);
+MctAppFilter *mct_app_filter_deserialize (GVariant      *variant,
+                                          uid_t          user_id,
+                                          GError       **error);
+
 /**
  * MctAppFilterBuilder:
  *