API: Add chat_template_kwargs alias for template_vars

This key is used in VLLM and SGLang.

Signed-off-by: kingbri <8082010+kingbri1@users.noreply.github.com>

docs/04.-Chat-Completions.md

@@ -23,6 +23,9 @@ A chat completions request to TabbyAPI also supports custom template variables i
 }
 ```
 
+> [!NOTE]
+> To preserve compatibility with other standards, `chat_template_kwargs` can be used instead of `template_vars`
+
 Now let's pass the custom var in the following template:
 ```jinja2
 I'm going to say {{ test_var }}

endpoints/OAI/types/chat_completion.py

@@ -1,4 +1,4 @@
-from pydantic import BaseModel, Field, field_validator
+from pydantic import AliasChoices, BaseModel, Field, field_validator
 from pydantic.json_schema import SkipJsonSchema
 from time import time
 from typing import Literal, Union, List, Optional, Dict
@@ -65,7 +65,11 @@ class ChatCompletionRequest(CommonCompletionRequest):
     messages: List[ChatCompletionMessage] = Field(default_factory=list)
     prompt_template: Optional[str] = None
     add_generation_prompt: Optional[bool] = True
-    template_vars: Optional[dict] = {}
+    template_vars: Optional[dict] = Field(
+        default={},
+        validation_alias=AliasChoices("template_vars", "chat_template_kwargs"),
+        description="Aliases: chat_template_kwargs",
+    )
     response_prefix: Optional[str] = None
     model: Optional[str] = None