Fix EZP-20608 eZRest v2 : Not possible to set fieldSetting "textRows" for contentfieldtype "eztext"

This PR fixes issue https://jira.ez.no/browse/EZP-20608

For parsing ```fieldSettings``` and ```validatorConfiguration``` ```FieldTypeProcessor``` is changed to abstract class and amended with new methods. New processors are implemented for FieldTypes requiring processing.
```FieldTypeProcessor``` implementation and tests are moved to ```REST/Common``` namespace, as they are used by both REST Client and Server (registry and interface were moved there previously).

REST specification is changed with new XSD definitions for ```fieldSettings``` and ```validatorConfiguration```, which are using already existing definition ```valueType```. Those are now analogous to existing ```fieldValueType``` definition. This means same rules now apply for them, in case of XML input of form ```<value key="someKey">someValue</value>``` is expected, which is consistent with response from REST server.