Support using bucket URLs without using a cache profile

[nstogner]

Currently bucket URLs (i.e. s3://, gcs://) are only supported in conjunction with .spec.cacheProfile. It would be nice to be able to load from those locations without needed to setup a cache profile.

Option A: Wait for vllm-project/vllm#10192 to land.
Option B: Load to disk/mem on the node, then point the serving engine at that mount path.

[nstogner]

Option B might be useful for engines other than vLLM. It is probably worthwhile checking to see if those engines already support loading directly from bucket URLs at this point.

[nstogner]

Complication with opt B is that nodes might not have sufficient storage/mem.