[Feature] add learnable agent

[rayrayraykk]

Class Description

The LearnableAgent class extends AgentBase and implements abstract base class (ABC) functionalities, serving as the core for creating agents capable of learning from textual interactions.

Key components of the LearnableAgent class include:

• reply: A method to be implemented by derived agents, allowing for customized response mechanisms.
• learn_from_chat: Processes messages from the agent's memory to learn and assess their value for knowledge preservation.
• archive_valuable_msg: Determines whether a message contains valuable knowledge and, if so, extracts a summary and stores it in the knowledge base.