Add Xgrammar Support

[flyinglandlord]

Change:

• change launch argument from --simple_constraint_mode to --output_constraint_mode, now user can choose the constriant decode backend from ['outlines', 'xgrammar']
• add XgrammarBackend used for xgrammar constraint decode, maybe we should merge it with SimpleConstraintBackend later?
• now we adopt the same request body, the same as vLLM with xgrammar(https://docs.vllm.ai/en/stable/serving/openai_compatible_server.html)
  • user can add guided_grammar to pass a EBNF grammar and execute the constraint decode
  • user can add guided_json to pass a standard json schema and do the constraint decode