[V1] Add structural_tag support using xgrammar (#17085)

vllm/sampling_params.py

@@ -38,6 +38,7 @@ class GuidedDecodingParams:
     """These are other options that can be set"""
     backend: Optional[str] = None
     whitespace_pattern: Optional[str] = None
+    structural_tag: Optional[str] = None
 
     @staticmethod
     def from_optional(
@@ -48,9 +49,10 @@ class GuidedDecodingParams:
         json_object: Optional[bool] = None,
         backend: Optional[str] = None,
         whitespace_pattern: Optional[str] = None,
+        structural_tag: Optional[str] = None,
     ) -> Optional["GuidedDecodingParams"]:
-        if all(arg is None
-               for arg in (json, regex, choice, grammar, json_object)):
+        if all(arg is None for arg in (json, regex, choice, grammar,
+                                       json_object, structural_tag)):
             return None
         # Extract json schemas from pydantic models
         if isinstance(json, (BaseModel, type(BaseModel))):
@@ -63,6 +65,7 @@ class GuidedDecodingParams:
             json_object=json_object,
             backend=backend,
             whitespace_pattern=whitespace_pattern,
+            structural_tag=structural_tag,
         )
 
     @property