[V1] Add logits_processors support

[imkero]

Add SamplingParams.logits_processors support for V1

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[imkero]

Maybe still some overhead near

parameters = inspect.signature(logits_processor).parameters

trying to enhance it

[njhill]

IMO we should take the V1 switch as an opportunity to change the design for logits processors. In particular I don't think we should support passing arbitrary LPs themselves per request (in SamplingParams) and rather be able to configure at startup time as plugins, which can consume custom request/sampling parameters.

Also I think we agreed that the interface should change to be stateful and work at a batch level (aligned with V1 persistent batch) so that vectorized implementations will be possible/encouraged by default.

[WoosukKwon]

+1 for @njhill's comment