Fix private to public
See https://github.com/huggingface/datasets-server/issues/380#issuecomment-1254740105:

> private -> public does not generate a webhook, it seems like a bug on the Hub. (btw: turning a public repo to private generates a "update" webhook, conversely)

and

https://github.com/huggingface/moon-landing/issues/2362#issuecomment-1254774183

> Meanwhile, in datasets-server, I will fix the issue by using the request (that generates the SplitsResponseNotFound error) to trigger a cache refresh if needed, so: not urgent

In this PR, if a response to /splits or /first-rows is NotFound but should have existed, ie. is a cache miss, we add a job to the queue, and return a NotReady error instead.
Fix private to public: See https://github.com/huggingface/datasets-server/issues/380#issuecomment-1254740105:

> private -> public does not generate a webhook, it seems like a bug on the Hub. (btw: turning a public repo to private generates a "update" webhook, conversely)

and

https://github.com/huggingface/moon-landing/issues/2362#issuecomment-1254774183

> Meanwhile, in datasets-server, I will fix the issue by using the request (that generates the SplitsResponseNotFound error) to trigger a cache refresh if needed, so: not urgent

In this PR, if a response to /splits or /first-rows is NotFound but should have existed, ie. is a cache miss, we add a job to the queue, and return a NotReady error instead.