Fix: PolarisFileSystem uses the dataset name instead of the dataset slug, causing a 404

[zhu0619]

Changelogs

• Sluggify the dataset name in the PolarisFileSystem.
• Serialize the cache_dir to a string.
• Updated Zarr dataset tutorial to reflect recent changes.

---

Checklist:

- [ ] Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
- [ ] Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
- [ ] Update the API documentation if a new function is added, or an existing one is deleted.

• Write concise and explanatory changelogs above.
• If possible, assign one of the following labels to the PR: feature, fix, chore, documentation or test (or ask a maintainer to do it for you).

---

When uploading the dataset, the dataset slug has to be used to get the correct path to the R2 bucket. Otherwise, the Hub returns a 404.

[cwognum]

Thanks @zhu0619 ! Just some minor suggestions.

[cwognum]

Hey @zhu0619 ! Seems I'm too late, but one more thought came to mind when I saw you push the latest changes: Do we need to sluggify the owner too? I don't think so, because even though we type-hint dataset_owner as a str, I believe it's actually a HubOwner object which returns its slug as string representation.

Maybe we should just drop the dataset_name and dataset_owner and use the artifact_id instead. It's only used for the prefix anyways. We can leave things as is for now!