Add fields `viewer` and `preview` to /is-valid
For coherence with /valid, we should add the `viewer` and `preview` fields to /is-valid.

We should also consider deprecating the current `valid` field (as in https://github.com/huggingface/datasets-server/issues/1445). Note that it's in use in https://github.com/search?q=org%3Ahuggingface+datasets-server.huggingface.co+repo%3Ahuggingface%2Fnotebooks&type=code and also in the @lewtun's evaluator if I remember correctly.
Add fields `viewer` and `preview` to /is-valid: For coherence with /valid, we should add the `viewer` and `preview` fields to /is-valid.

We should also consider deprecating the current `valid` field (as in https://github.com/huggingface/datasets-server/issues/1445). Note that it's in use in https://github.com/search?q=org%3Ahuggingface+datasets-server.huggingface.co+repo%3Ahuggingface%2Fnotebooks&type=code and also in the @lewtun's evaluator if I remember correctly.