HPCC-9595 WIP - Revise based on code review

1. Add ContentType for each file to indicate a key/csv/xml/flat file;
2. Rename IsKeyFile to IsCompressed (keep old one for legacy);
3. Change the type of logicalFileSortFields[] from mapEnums to const
char* array;
4. Fix a typo.

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>