WIP: Vine Shared Filesystem Access via Symlinks

[dthain]

Proposed Changes

Add capability to add symlink to task sandbox namespace.
Will then add mode to substitute regular file objects with symlinks to shared filesystem.

Merge Checklist

The following items must be completed before PRs can be merge.
Check these off to verify you have completed all steps.

• make test Run local tests prior to pushing.
• make format Format source code to comply with lint policies. Note that some lint errors can only be resolved manually (e.g., Python)
• make lint Run lint on source code prior to pushing.
• Manual Update Update the manual to reflect user-visible changes.
• Type Labels Select a github label for the type: bugfix, enhancement, etc.
• Product Labels Select a github label for the product: TaskVine, Makeflow, etc.
• PR RTM Mark your PR as ready to merge.