Add metadata field to basemessage (#5372)

Add metadata field to BaseMessage.

Why?
- additional metadata field can track 1) timestamp if needed, 2) flags
about the message. For instance, a use case is a metadata field
{"internal":"yes"} that would hide messages from being displayed in an
application or studio.

As long as an extra field is added to basemessage that is not consumed
by existing agents, I am happy.



Notes:
- We can also only add it to BaseChatMessage, that would be fine
- I don't care what the extra field is called as long as there is an
extra field somewhere
- I don't have preference for the type, a str could work, but a dict
would be more useful.

---------

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>

python/packages/autogen-agentchat/src/autogen_agentchat/messages.py

@@ -5,7 +5,7 @@ class and includes specific fields relevant to the type of message being sent.
 """
 
 from abc import ABC
-from typing import List, Literal
+from typing import Dict, List, Literal
 
 from autogen_core import FunctionCall, Image
 from autogen_core.memory import MemoryContent
@@ -23,6 +23,9 @@ class BaseMessage(BaseModel, ABC):
     models_usage: RequestUsage | None = None
     """The model client usage incurred when producing this message."""
 
+    metadata: Dict[str, str] = {}
+    """Additional metadata about the message."""
+
     model_config = ConfigDict(arbitrary_types_allowed=True)