HPCC-11116 add keyed field indicator to fvresult xml schema

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #5820 from afishbeck/xmlSchemaIndicateKeyedFields

HPCC-11116 add keyed field indicator to fvresult xml schema

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>