[Core] Remove external storage upon sigterm for ray start

[ruisearch42]

Why are these changes needed?

Currently, spilled directories are destroyed only when driver cluster (ray.init) is terminated, but not cluster started by ray start.

This PR fixes the issue by always removing external storage upon SIGTERM by calling IO worker.

Related issue number

Closes #17790

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    method in Tune, I've added it in doc/source/tune/api/ under the
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• Testing Strategy
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  • This PR is not tested :(

[rkooo567]

I think we should make the call blocking & have some timeout (maybe 10 seconds or sth) so that the callback part is not messed up with shutdown path

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