store ingested tarball artifacts (in a separate storage area)

We want to have a dedicated storage where to store ingested tarball and similar artifacts (e.g., Debian source packages).

This way we will be able to return bit-identical artifacts upon demand even when they are not (bit-identical) reproducible from our data model. Note that this is actually the case for tarballs, due to all the very advanced metadata about ownership/permissions that they can store.

By doing this we will also ensure that the checksum references stored in revision.metadata are not dangling in our global data model/storage.

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Migrated from T341 (view on Phabricator)