Add validate_table_existence to abstract SettingsManager (#5236)

The `BackendManager` was using the `validate_table_existence` from its
`SettingsManager` member, but this method was not defined in the abstract
class. It was present in both backends implementations, but there would
be nothing requiring from the abstract parent class.

Co-authored-by: Chris Sewell <chrisj_sewell@hotmail.com>

aiida/backends/manager.py

@@ -94,6 +94,13 @@ def delete(self, key):
         :raises: `~aiida.common.exceptions.NotExistent` if the settings does not exist
         """
 
+    @abc.abstractmethod
+    def validate_table_existence(self):
+        """Verify that the `DbSetting` table actually exists.
+
+        :raises: `~aiida.common.exceptions.NotExistent` if the settings table does not exist
+        """
+
 
 class BackendManager:
     """Class to manage the database schema and environment."""