FILE_SEG filename option - NIXLBench Support

[aranadive]

@lluki please add support for this option in the PR in the CI tests - .gitlab/test_cpp.sh.
Also add nixlbench support for this one. We will need QA to test the path option correctly. Its a new feature for FILE_SEG API - separate PR is fine.

Originally posted by @aranadive in #1635 (comment)

[lluki]

Functional testing is already included in the original PR, lets make this issue and follow up PR solely on the nixlbench modifications