Adds an API route for fetching objects as Git objects

This route is needed for the implementation of the SWH Archive-IPFS
bridge. Specifically, we need a way to, given a SWHID, fetch the exact
data that was used to hash an object.

The design follows the plan @vlorentz put forth in
https://sympa.inria.fr/sympa/arc/swh-devel/2022-04/msg00042.html