Implemented EZP-21099: refactor REST FieldTypeProcessors to use service tags

See https://jira.ez.no/browse/EZP-21101 (subtask of https://jira.ez.no/browse/EZP-20829).

This PR implements field type processors (process input & output custom field type data) to be instanciated using service tags instead of a hardcoded factory.

Note: this most like totally breaks the REST client tests. @pspanja ? :-)