allow exporting to tar files

Summary:
Sometimes we want to dump a larger portion of our monorepo. `scsc export` is fine method of doing so but it has to be aware of local filesystem limitations: number of inodes, case sensitivity etc etc. It's also harder to copy/move many small files.

**Exporting straight to tar file** solves the problem. A single file is easy to manage and immune to local filesystem peculiarities.

**Tradeoff:** I've had to rearchitect the export to be more sequential (as write to tar files is sequential in its nature). It might impact the perf of large exports to fs. I mostly mitigated it by more buffering but there's still some penatly.

Reviewed By: clara-9

Differential Revision: D67803881

fbshipit-source-id: 5b8b1c328a06a8e00c1e35f372e8a30ccc2b47c3