server: add ctx-per-slot (--kv-unified-per-slot) (#24124)
* Add ctx-per-slot argument for unifid KV cache
* Swap out ctx fractions for ctx pool slots
* Formatting cleanup
* Remove ctx-pool-slots, make ctx-per-slot an int
* refactor it
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Co-authored-by: Xuan Son Nguyen <son@huggingface.co>
