Make the FieldType available as a property of a Content's Fields.

As discussed in EZP-28253, the type of a field is very often needed, both over REST and PHP. Furthermore, the FieldTypes are already loaded during the building of a Content value object. Adding them wouldn't add any new dependency or load operation, nor require extra cache purge.

The suggested name is typeIdentifier, as "field" is already part of the original value object's name.
// Edit: updated suggested name due to review conclusions.