base-sim: allow explicit wait for SIM readiness during initialization

Before attempting to load any SIM property value, allow checking
whether the SIM is ready for operation or not.

This action is implicitly done by the "unlock required check" step
that is triggered before initializing the primary SIM card, but it
would not be done when initializing other available SIM cards.

src/mm-base-sim.h

@@ -52,6 +52,14 @@ struct _MMBaseSim {
 struct _MMBaseSimClass {
     MmGdbusSimSkeletonClass parent;
 
+    /* Wait SIM ready (async) */
+    void     (* wait_sim_ready)        (MMBaseSim            *self,
+                                        GAsyncReadyCallback   callback,
+                                        gpointer              user_data);
+    gboolean (* wait_sim_ready_finish) (MMBaseSim            *self,
+                                        GAsyncResult         *res,
+                                        GError              **error);
+
     /* Load SIM identifier (async) */
     void    (* load_sim_identifier)        (MMBaseSim *self,
                                             GAsyncReadyCallback callback,