feat: async memory thread

[vivek3141]

This PR adds a SplitThreadAgent. This agent uses a separate memory_agent to update the memory of the conversation_agent in the background. At each step, the conversation_agent has the ability to pick a tool wait_for_memory_update, which allows it to wait for the memory to be fully updated before proceeding. Functionality is tested with tests/test_split_thread_agent.py.