Add an Image artifact type #1415
Description
Is your feature request related to a problem? Please describe.
When creating the experiment tracking UI based on the APIs in #1318, it'd be nice if there was an artifact type of Image so that users can upload custom visualizations from their runs to be rendered in the experiment tracking UI.
Describe the solution you'd like
A new artifact type of Image with name, URI, and metadata.
Describe alternatives you've considered
We could reuse something generic like a DocArtifact and use a special custom property to indicate to the UI that it's renderable.