[WIP] Added ability to set translation domain on field description

This PR allows a translation domain to be specified on a field description via. `$options['translation_domain']`.

The TWIG form template uses the `field_description.translationDomain` which will first check to see if a specific domain has been set for this field, if not it will fall back to the Admin domain.

The use case is for the new Extension feature - in the CMF we want to be able to define some extensions in the CoreBundle which apply to Admin classes in various bundles with various translation domains, obviously it is important that we can somehow specify the CoreBundle as the translation domain for fields added using the CoreBundle extension.

This hasn't been (manually) tested properly yet.