Fix EZP-21035: User can not be properly created from inside storage

This PR resolves issue https://jira.ez.no/browse/EZP-21035

Check for `isSingular` feature of the FieldType was missing from `createContentType()` method, therefore making possible to create ContentType with two singular fields. In `createContentType()` this check is handled by new `ContentTypeValidationException` and in `addFieldDefinition()` it is handled by `BadStateException`.

Also found and fixed some additional problems:

* FieldDefinition validation was throwing exception on first invalid definition, while it should return validation errors on all of them
* Added `ContentTypeFieldDefinitionValidationException` to the `addFieldDefinition()` method interface. This exception was already thrown from there, so the change here is just syncing the interface (plus fix from the first point).

REST specification and BC changes for 5.2 are updated.

### TODO

- [x] add a FieldType property for ability to add a FieldType to a content type with existing instances