HPCC-11922 DESDL add JSON Support

Add rewindTo IXmlWriterExt method
Add JSON builder, JSON test
Add JSON helper functions to esp/services/common

Signed-off-by: rpastrana <rodrigo.pastrana@lexisnexis.com>
#HPCC-PLATFORM-11922 - changes from code review
- IXmlWriterExt.rewindTo() not valid for flusher type
- moved wsexcept from LN/esp/esdllib to esp/services/common
- moved non json specific logic from JsonHelpers to HttpParamHelpers
- cleared tmp strbuffer upon every appendjsonexceptions iteration
- moved buildJsonMsg from ws_ecl to JsonHelpers
- ws_ecl now utilizes JsonHelpers
Signed-off-by: rpastrana <rodrigo.pastrana@lexisnexis.com>
HPCC-11922-NEW-JSON-Support changes from code review
- substitute all static method class for namespace scoping
- refactor odd member field nomenclature

Signed-off-by: rpastrana <rodrigo.pastrana@lexisnexis.com>
HPCC-11922-NEW-JSON-Support more changes from code review
- refactor member vars

Signed-off-by: rpastrana <rodrigo.pastrana@lexisnexis.com>