[prototype] prioritized block soft pinning/evictions

[simon-mo]

built on top of #17519, this PR showcase a way to prioritized KV cache blocks by using the request priority flag. the usage is to keep a notion of priority in the free queue so some blocks are freed earlier than others, instead of a global LRU ordering.

see examples/experiments/offline-priority-prefix-caching.py and examples/experiments/online-priority-prefix-caching.py for usage. in short the request priority needs to be set as priority: 1 or 0. note we are just abusing the API here that this doesn't implement priority scheduling

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[mergify]

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merged. Please rebase the PR, @simon-mo.

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