Implement an object storage scrubber

In order to check the contents stored in object storages can be found and are valid, we need to do the following:

• Get the full list of content hashes referenced in SWH storages using kafka or partitions of contents from storage API
• For each content:
  • check its existence in all object storages (S3, azure, disk based, winery)
  • download its bytes, recompute the hashes and check we obtain the same

Related to https://gitlab.softwareheritage.org/product-management/core-platform/-/work_items/23