Mark annotation field as computed

I reviewed the existing code and it makes sens. The field does not need to be stored in Drupal DB.
I added ->setComputed(TRUE) to the xntt annotation field (ExternalEntity.php) and made basic tests and it seems to work fine, no error, no logs, expected behavior on both annotation content and annotated external entity instances.
Committed to v3.

i have created the patch please review it.