Add `JsonArtifact`

[vachillo]

• [x] I have read and agree to the contributing guidelines.

Is your feature request related to a problem? Please describe. I want a way to pass around json-serialized values without having to explicitly de/serialize the dict myself

Describe the solution you'd like JsonArtifact that takes a dict and serializes to json.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.