add missing "length" property to revision.metadata JSON for all imported tar/zip

the original_artifact JSON properties in revision.metadata for all imported tar/zip files look this:

{
  "original_artifact" : [
    {
      "name" : "dejagnu.texi.tar.gz",
      "sha1" : "9a5380aa7c2a9fb7f84036b223cbde07d9db7e67",
      "sha256" : "68f5a356bae22bfef8d09c6314d2c508016507b197e298bcb210a2cdf674cb93",
      "sha1_git" : "26a738cc74f236e7e2e1ee1a00817dfe5716aa2d",
      "archive_type" : "tar"
    }
  ]
}

A length field is missing and we should add it, both to the tarball loader and to the already imported data.

Rationale: the lack of a length field is inconsistent both with the layout of the content table and with what the git loader does.

Note that the expected JSON Schema is now available in swh-storage under sql/json/.

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Migrated from T339 (view on Phabricator)