Add model to pre-migrate "lazy catalog" from pulp2
Pulp2LazyCatalog - importer_id - unit_id - content_type - url - checksum - checksum_algorithm
Expand the content migration pipeline to create ContentArtifacts and RemoteArtifatcs.
Since it's possible to migrate content only. Ensure that on_demand content is taken care of only when Remote is migrated/available. Otherwise, only downloaded content should be migrated.