Support general artifact type

Current [artifactType](https://github.com/google/ml-metadata/blob/master/ml_metadata/proto/metadata_store.proto#L55) can't be nested object and the property types can only be int, double or string. This is inconvenient to use. E.g. one may want to define a property for `annotation` of type map.

Instead of defining a type system in protobuf, is it possible to use other dedicated schema definition mechanisms such as OpenAPI schema or Apache Avro? [Protobuf doesn't seem to be the right tool to validate schema during runtime.](https://github.com/protocolbuffers/protobuf/issues/3600#issuecomment-329266055)