Improve Performance: CoPilot: users experience

[quge009]

This PR is mainly about improving the user experience.

• Changes made to optimize users perceived latency and reading experience:
  • Implement streaming output for LLMSession class, change the final answer generation call to streaming output, to post the answer to user as-soon-as the first few tokens are ready.
  • Implement the push_frontend method to leverage the steaming output to feedback the CoPilot progress status message to user in real-time, to manage users' experience during waiting for the answer.
  • Add auto scroll feature for frontend plugin to enhance readability.
• Changes made to reduce the average_response_latency (defined as time between question receival and answer posting):
  • Refactor several components' (SmartHelp, LTP, ...) implementation into classes, to make it possible to preserve the states when necessary.
  • Reuse the same llm_session instance for requests within the same conversation, by avoiding unnecessary https re-connection in initialization.
  • Implement a new question parsing function to combine contextualization and classification llm calls into one efficient call, to reduce time.
  • Move prompt reading to instance initialization, to avoid unnecessary file I/O operations.
• Also a minor bug fix is included:
  • Change the assignment of 'turnId' to frontend.

Effectiveness of this PR:

• Impact on accuracy
  • No change
• Impact on response latency
  • ~15% response time reduction on average
  • ~50% response time reduction for extreme simple question