Make the offloading blobs PoC MR work with GCP.

POC expectations (Updated on 2023-04-18)

The POC can be perform on GDK, with modified Gitaly binary as the Gitaly backend. We are expecting to see:

1. A user can use Rails command or Web UI of housekeeping to trigger the offloading process.
2. On the server side, Gitaly will repack the repository based on the filter setup.
3. On the server side, Gitaly will upload the large blob packs onto cloud storage (e.g GCP buckets).
4. On the client side, the repo can be cloned, fully functional without missing objects.
5. On the client side, offloaded packs are downloaded directly from cloud storage (not through gitaly server).

Some extra expectation if possible:

1. A user can explore the large object on gitlab UI without downloading the blob. This require cat-file to be modified to recognize the offloaded objects and can streaming it metadata.