[Artifacts] Fix Import Project with remote Artifacts

[quaark]

https://iguazio.atlassian.net/browse/ML-6466

When exporting a project with remote(non-inline) artifacts, they get exported in the structure:

{
  "key": "<artifact key>",
  "import_from": "<object store url>",
}

Therefore it does not match the artifact schema with metadata, spec, status and fails on trying to extract metadata.key when it can extract key directly.

Fixed the object validation during import to support remote artifacts.