feat: allow handle/response of a ToolMessage to return an arbitrary type of result, or an enabled/non-enabled ToolMessage.

(This supersedes 0.9.0 release notes)

• tests: See test_tool_handlers_and_results in test_tool_messages.py
• implementation: handle_tool_message in agent/base.py
• example: https://github.com/langroid/langroid/blob/main/examples/basic/tool-extract-short-example.py

The result R of a handle/response method of a ToolMessage ends up either in the content field (as a string) of the returned ChatDocument, OR in the tool_messages field of the returned ChatDocument. This is determined as follows:

• if R is already a string OR ChatDocument, it is returned as is (=> content field)
• else if R is a ToolMessage:
  • if it is a "handleable" tool message, its handler method is called recursively
  • else it is added to the tool_messages field of the returned ChatDocument
    • A special case of this is if R is an instance of FinalResultTool: this acts as a "short-circuit termination", i.e.:
      • (a) the current task as well as all parent tasks up to the root task are terminated, and
      • (b) the result R appears in the final returned ChatDocument's tool_messages list
• else if R is a pydantic object (i.e. derived from BaseModel), it is converted to a string using obj.json() (=> content field)
• else an attempt is made to convert it to string using json.dumps(R) or str(R) (=> content field)