Add way to pass construct time options to plugin

This is needed to allow tests to manipulate the behavior of the test
shell plugin during startup. Since the plugin is created and started
when the MetaDisplay is created, it needs to be handled via MetaContext,
by setting the options after creating the context, but before starting.

For simplicity reasons, make the options an opaque GVariant, passed via
a an `"options"` property when the plugin object is created, if the
passed options is non-NULL. Only passing the options when non-NULL
allows for backward compatibility.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4152>

src/core/meta-context-private.h

@@ -91,3 +91,7 @@ void meta_context_set_trace_file (MetaContext *context,
 #endif
 
 MetaSessionManager * meta_context_get_session_manager (MetaContext *context);
+
+META_EXPORT_TEST
+void meta_context_set_plugin_options (MetaContext *context,
+                                      GVariant    *plugin_options);