Add support for symbolic links

[dpastoor]

There isn't really a need for many of these files to actually have them in the repository itself, rather could instead just link. We'd need to protect the file from being modified though by the user so in the process of linking would want to set the file to read-only. This could cause confusion to users if they later try to modify since they'd get generic permissions errors

impact assessment

• status/ls would need to follow symlinks to original file
• mechanism for user to know which files from dvs were linked vs copies
• undo or swap link to original
• IO considerations - if the project dir itself is on one FS vs the original data on separate one, this could cause performance changes in the repo activities itself based on where subsuquent read IO would be handled against
• migration of dvs root directories would be much more risky

implementation timeline

for now just tracking this idea, no need to implement unless the burden of copying files becomes a common point of feedback

[andriygm]

would this feature symlink project files to the files in storage? if we do S3 we may want to symlink the project files instead to the cache, and dvs get into said cache instead (I believe this is how dvc works as well but I could be mistaken)