Add a "generic" artifact type

[mprahl]

Is your feature request related to a problem? Please describe.

I'm currently investigating how Kubeflow Pipelines can integrate with the experiment tracking APIs in #1318.

For context, Kubeflow Pipelines has the following output artifact types:

• Artifact - Generic artifact if there isn't a native type.
• Dataset
• Model
• Metrics
• ClassificationMetrics
• HTML and Markdown

I think the main one that doesn't have a specific mapping to the experiment tracking APIs in #1318 is the generic artifact.

Describe the solution you'd like

Add a "generic" artifact type to be used when there isn't a native type in Model Registry. It simply has a name, URI, and metadata. This a type that has no rendering in the UI and is mostly to record an output associated with a run.

Describe alternatives you've considered

We could reuse something generic like a DocArtifact but the concern is that any UI experience may try to render such an artifact and it may be confusing.