feat: Add metadata field to request and response

This changes adds Optional metadata field to OpenAI compatible request and
response object.

fixes: #3564
Signed-off-by: Abhishek Bongale <abhishekbongale@outlook.com>

src/llama_stack_api/agents.py

@@ -89,6 +89,7 @@ class Agents(Protocol):
             ),
         ] = None,
         max_tool_calls: int | None = None,
+        metadata: dict[str, str] | None = None,
     ) -> OpenAIResponseObject | AsyncIterator[OpenAIResponseObjectStream]:
         """Create a model response.
 
@@ -100,6 +101,7 @@ class Agents(Protocol):
         :param include: (Optional) Additional fields to include in the response.
         :param guardrails: (Optional) List of guardrails to apply during response generation. Can be guardrail IDs (strings) or guardrail specifications.
         :param max_tool_calls: (Optional) Max number of total calls to built-in tools that can be processed in a response.
+        :param metadata: (Optional) Dictionary of metadata key-value pairs to attach to the response.
         :returns: An OpenAIResponseObject.
         """
         ...

src/llama_stack_api/openai_responses.py

@@ -597,6 +597,7 @@ class OpenAIResponseObject(BaseModel):
     :param usage: (Optional) Token usage information for the response
     :param instructions: (Optional) System message inserted into the model's context
     :param max_tool_calls: (Optional) Max number of total calls to built-in tools that can be processed in a response
+    :param metadata: (Optional) Dictionary of metadata key-value pairs
     """
 
     created_at: int
@@ -619,6 +620,7 @@ class OpenAIResponseObject(BaseModel):
     usage: OpenAIResponseUsage | None = None
     instructions: str | None = None
     max_tool_calls: int | None = None
+    metadata: dict[str, str] | None = None
 
 
 @json_schema_type