[Feature] integrate Structural Tag in xgrammar backend for function calling (#3566)

Co-authored-by: shuaills <shishuaiuoe@gmail.com>

python/sglang/srt/sampling/sampling_params.py

@@ -45,6 +45,7 @@ class SamplingParams:
         json_schema: Optional[str] = None,
         regex: Optional[str] = None,
         ebnf: Optional[str] = None,
+        structural_tag: Optional[str] = None,
         no_stop_trim: bool = False,
         ignore_eos: bool = False,
         skip_special_tokens: bool = True,
@@ -72,6 +73,7 @@ class SamplingParams:
         self.n = n
         self.json_schema = json_schema
         self.ebnf = ebnf
+        self.structural_tag = structural_tag
         self.no_stop_trim = no_stop_trim
         self.return_hidden_states = return_hidden_states
         self.custom_params = custom_params