[IMP] google_calendar: add google sync helper

Add a helper method to know when the user calendar
is fully synced to Google Calendar, meaning when
API calls can correctly be performed.
The user calendar is considered fully synced if the
Google API is correctly configured in the settings
(Client ID / Secret) and the user has synced its
calendar with its Google Account.

Task-3285601

Part-of: odoo/odoo#133921

addons/google_calendar/models/res_users.py

@@ -133,6 +133,12 @@ class User(models.Model):
                 _logger.exception("[%s] Calendar Synchro - Exception : %s!", user, exception_to_unicode(e))
                 self.env.cr.rollback()
 
+    def is_google_calendar_synced(self):
+        """ True if Google Calendar settings are filled (Client ID / Secret) and user calendar is synced
+        meaning we can make API calls, false otherwise."""
+        self.ensure_one()
+        return self.google_calendar_token and self._get_google_sync_status() == 'sync_active'
+
     def stop_google_synchronization(self):
         self.ensure_one()
         self.google_synchronization_stopped = True