[Feature] Support a custom logit processor

[merrymercy]

We would like to support custom logit processors registered by users, so users can easily implement their custom sampling methods without the need to change the sglang code.

Steps

1. After the following line, allowing running some user code to process the logits
   

   sglang/python/sglang/srt/layers/sampler.py

   Line 37 in 5f12f0e

   logits = logits.contiguous()

2. Add a new field custom_params to SamplingParams

   sglang/python/sglang/srt/sampling/sampling_params.py

   Line 43 in 9a00e6f

   skip_special_tokens: bool = True,

   so users can pass in their arguments.

See also huggingface/transformers#14779

[zhyncs]

BTW, constrained decoding and function calling can also be implemented this way, such as by crafting a mask kernel.
The schema part can continue to use outlines or xgrammar.

[zhyncs]

ref https://www.youtube.com/watch?v=I-a8sTMUq5o&t=3111s

[zhyncs]

cc @Ubospica

[hongpeng-guo]

May I take this issue if no one is looking at it rn?