Feature Request: Add ability to change file permissions on imported files

[brentleyjones]

We should have an option similar to rsync's --chmod to change imported record and unit permissions. A common use case for index-import is importing Bazel built index stores. Those records are read-only, so users have to manually adjust them after the import with a command similar to chmod -R u+w "$INDEX_DATA_STORE_DIR". This is inefficient, as it does it to all files, instead of just the records that are imported.

And since we write units new each time, the flag could be useful to do the opposite: make all files imported read-only (if someone wanted to do that for some reason).