POC: Gitlab Migration - compress exported relations using zip

What does this MR do and why?

This MR updates Gitlab Migration to compress exported relations using zip instead of gz (and tar for archiving). It brings a few benefits:

1. Less system calls. We can do all zipping with ruby
2. We can read individual file contents directly from object storage instead of downloading the whole file (in the future, right now this POC downloads the whole file). This can be beneficial when importing a collection of heavy uploads that take a lot of disk space

In order to implement 2 & download individual files directly from object storage we need to know their filenames upfront, which would require additional changes to exported data (e.g. having metadata.json describing exported files, their filenames and sizes so that we can read them from object storage). We can iterate on it once we have transitioned to zip.

Mentions #376261 (closed)

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