Enable planning in AssistantAgent

The AssistantAgent should exhibit planning capabilities in case of complicated tasks. Some key functionalities to add:
- Goal Setting: The agent should be able to define clear and specific objectives based on the task at hand. It may involve deriving subtasks from a complex task.
- Goal Checking (Monitoring and Reviewing): This involves tracking progress toward the objectives, adjusting the plan as necessary, and reviewing the effectiveness of the plan after it's been implemented. A reflection agent similar to https://github.com/microsoft/FLAML/pull/1019/files#diff-1c33de07bece8926d2bc1edd0dd4a31d7234105d3f3c8cc740a92c4a16668d55 might be needed.
- Strategy devising and adaption: This involves adapting the strategy based on changing conditions.