Reconstruct Artifact object from MLMD proto descriptors.

Internal method, no backwards compatibility guarantees.

artifact_type A metadata_store_pb2.ArtifactType proto object describing the type of the artifact.
artifact A metadata_store_pb2.Artifact proto object describing the contents of the artifact. If not provided, an Artifact of the desired type with empty contents is created.

Artifact subclass object for the given MLMD proto descriptors.