Add copilot endpoint

[y1811]

Add copilot endpoint support. We can configure if use endpoint in [pfcopilot.env].

• Flow link
• Endpoint link

In this PR:

• we separate original copilot_context.py into two files: copilot_management_context.py and copilot_gpt_context.py.
  • copilot_management_context.py is responsible for entire chat context, and have dump flow/read folder related logic.
  • copilot_gpt_context.py is responsible for communicating with gpt.
• we support stream output in endpoint scenario.

Reference doc: https://microsoft.github.io/promptflow/how-to-guides/enable-streaming-mode.html#create-a-streaming-enabled-flow