WIP Vine Shared Filesystem Mode

[dthain]

Proposed Changes

Added new access mode enabled by vine_tune() to compare TaskVine caching behavior to direct use of shared filesystem.

• Add VINE_SHAREDFS file type which causes worker to link from sandbo…x to shared filesystem.
• Added shared_filesystem_link_mode to replace VINE_FILE with VINE_SHAREDFS.
• Rename load_from_shared_fs to shared_filesystem_cache_mode, which replaces VINE_FILE with VINE_URL.

Merge Checklist

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• 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.
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