Added the database module storage adapter.

[zinigor]

This is an adapter that allows the database-module to be used as the PluralObject storage option:

        $storage = new DatabaseModuleStorage( 'tdb_table_name' );

        $storage->register( 'tdb_table_name', [
            'schema' => [
                'id' => [
                    'type'           => 'bigint',
                    'length'         => '20',
                    'auto_increment' => true,
                    'primary_key'    => true,
                ],
                'title' => [
                    'type'   => 'varchar',
                    'length' => '255',
                ],
            ],
        ] );

        $dataset = new DataSet();
        $dataset->add_string( 'title' );

        $object = new PluralObject( 'tdb_plural_list', $storage );
        $object->set_dataset( $dataset );