[Feature] Support llguidance for constrained decoding

[JC1DA]

Motivation

This pull request integrates llguidance backend to extend guided decoding capabilities for sglang llguidance backend supports regex, json and grammar (lark or ebnf)

We have just released a large JSON Schema benchmark and a paper. Of particular interest might be isolated mask-generation benchmarks - comparing LLGuidance, Outlines, XGrammar and llama.cpp grammars.

Modifications

• Added llguidance constrained backend to support masked decoding & json/ebnf/regex testcases.
• We have plan to further support fast-forward decoding in the near future

Checklist

• [x] Format your code according to the Code Formatting with Pre-Commit.
• [x] Add unit tests as outlined in the Running Unit Tests.
• [ ] Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.