[Feature] Expose a new function for FileSegmentRef

[Erik Kaneda]

Feature

At the time of writing, we don't allow users to call FileSegmentRef::new(...) because we don't export the SegmentPath type. By making SegmentPath visible or better - creating a second new function that takes PathBuf will allow users to create instances of FileSegmentRef.

Motivation

Some users implement their own version of FileSegmentRef and this will allow them to use the risc0-zkvm's.

Implementation

This was described in the first section

[linear]

ZKVM-372 [Feature] Expose a new function for FileSegmentRef

[Erik Kaneda]

Expose nullFilesegment ref