api,simple: new 'multiplex' setting in bearer properties

Both the Simple.Connect() and Modem.CreateBearer() are updated to
allow a new 'multiplex' setting in the properties provided by the user
in both of these methods.

The new setting expects a MMBearerMultiplexSupport enum indicating
what kind of multiplex needs the user has:
 * none: if multiplex must not be used.
 * requested: if multiplex should be used if available.
 * required: if multiplex must be used.

The underlying implementations will take care of accepting or
rejecting the setting depending on the system and modem capabilities.

src/mm-bearer-mbim.c

@@ -1125,6 +1125,14 @@ _connect (MMBaseBearer *self,
     if (!peek_ports (self, &device, &data, callback, user_data))
         return;
 
+    if (mm_bearer_properties_get_multiplex (mm_base_bearer_peek_config (self)) == MM_BEARER_MULTIPLEX_SUPPORT_REQUIRED) {
+        g_task_report_new_error (
+            self, callback, user_data, _connect,
+            MM_CORE_ERROR, MM_CORE_ERROR_UNSUPPORTED,
+            "Multiplex support not available");
+        return;
+    }
+
     g_object_get (self,
                   MM_BASE_BEARER_MODEM, &modem,
                   NULL);

src/mm-bearer-qmi.c

@@ -1692,6 +1692,14 @@ _connect (MMBaseBearer *_self,
     GTask *task;
     GCancellable *operation_cancellable = NULL;
 
+    if (mm_bearer_properties_get_multiplex (mm_base_bearer_peek_config (_self)) == MM_BEARER_MULTIPLEX_SUPPORT_REQUIRED) {
+        g_task_report_new_error (
+            self, callback, user_data, _connect,
+            MM_CORE_ERROR, MM_CORE_ERROR_UNSUPPORTED,
+            "Multiplex support not available");
+        goto out;
+    }
+
     g_object_get (self,
                   MM_BASE_BEARER_MODEM, &modem,
                   NULL);