[FEAT] JSON constrained support (#1125)

Co-authored-by: Yineng Zhang <me@zhyncs.com>

docs/en/sampling_params.md

@@ -60,6 +60,9 @@ spaces_between_special_tokens: bool = True,
 regex: Optional[str] = None,
 # Do parallel sampling and return `n` outputs.
 n: int = 1,
+# Constrains the output to follow a given JSON schema.
+# `regex` and `json_schema` cannot be set at the same time.
+json_schema: Optional[str] = None,
 
 ## Penalties. See [Performance Implications on Penalties] section below for more informations.