Fix EZP-23226: Selection custom attributes on embed tags are not stored

The feature, which is documented online and in content.ini, is as far as I can tell simply missing in the code. Added here, based on similar code in https://github.com/ezsystems/ezpublish-legacy/blob/f2fc838a/extension/ezoe/ezxmltext/handlers/input/ezoexmlinput.php#L1873 and https://github.com/ezsystems/ezpublish-legacy/blob/f2fc838a/extension/ezoe/ezxmltext/handlers/input/ezoexmlinput.php#L1902

https://jira.ez.no/browse/EZP-23226