String trim flag when reading hpcc files

Instead of doing a post-read string trim on HPCC STRINGXX fields, it would be faster to have the record reader do it.

can it have a couple of additional flags along the lines of the unsigned8 change recently made:

1. A flag saying that if a field is a STRINGXX field (as opposed to variable STRING), trim the value when returning it
2. A flag saying that if a STRING-ish field is an empty string, convert the value to null